Working with File objects in AIR

A File object is a pointer to a file or directory in the file system.

The File class extends the FileReference class. The FileReference class, which is available in Adobe® Flash® Player as well as AIR, represents a pointer to a file. The File class adds properties and methods that are not exposed in Flash Player (in a SWF file running in a browser), due to security considerations.

About the File class

You can use the File class for the following:

• Getting the path to special directories, including the user directory, the user's documents directory, the directory from which the application was launched, and the application directory

• Coping files and directories

• Moving files and directories

• Deleting files and directories (or moving them to the trash)

• Listing files and directories contained in a directory

• Creating temporary files and folders

Once a File object points to a file path, you can use it to read and write file data, using the FileStream class.

A File object can point to the path of a file or directory that does not yet exist. You can use such a File object in creating a file or directory.

Paths of File objects

Each File object has two properties that each define its path:

Property	Description
nativePath	Specifies the platform-specific path to a file. For example, on Windows a path might be "c:\Sample directory\test.txt" whereas on Mac OS it could be "/Sample directory/test.txt". A nativePath property uses the backslash (\ character as the directory separator character on Windows, and it uses the forward slash (/) character on Mac OS and Linux.
url	This may use the file URL scheme to point to a file. For example, on Windows a path might be "file:///c:/Sample%20directory/test.txt" whereas on Mac OS it could be "file:///Sample%20directory/test.txt". The runtime includes other special URL schemes besides file and are described in Supported AIR URL schemes

The File class includes static properties for pointing to standard directories on Mac OS, Windows, and Linux. These properties include:

• File.applicationStorageDirectory —a storage directory unique to each installed AIR application. This directory is an appropriate place to store dynamic application assets and user preferences. Consider storing large amounts of data elsewhere.

  On Android and iOS, the application storage directory is removed when the application is uninstalled or the user chooses to clear application data, but this is not the case on other platforms.

• File.applicationDirectory —the directory where the application is installed (along with any installed assets). On some operating systems, the application is stored in a single package file rather than a physical directory. In this case, the contents may not be accessible using the native path. The application directory is read-only.

• File.desktopDirectory —the user's desktop directory. If a platform does not define a desktop directory, another location on the file system is used.

• File.documentsDirectory —the user's documents directory. If a platform does not define a documents directory, another location on the file system is used.

• File.userDirectory —the user directory. If a platform does not define a user directory, another location on the file system is used.

Note: When a platform does not define standard locations for desktop, documents, or user directories, File.documentsDirectory, File.desktopDirectory, and File.userDirectory can reference the same directory.

These properties have different values on different operating systems. For example, Mac and Windows each have a different native path to the user's desktop directory. However, the File.desktopDirectory property points to an appropriate directory path on every platform. To write applications that work well across platforms, use these properties as the basis for referencing other directories and files used by the application. Then use the resolvePath() method to refine the path. For example, this code points to the preferences.xml file in the application storage directory:

var prefsFile:File = File.applicationStorageDirectory;
prefsFile = prefsFile.resolvePath("preferences.xml");

Although the File class lets you point to a specific file path, doing so can lead to applications that do not work across platforms. For example, the path C:\Documents and Settings\joe\ only works on Windows. For these reasons, it is best to use the static properties of the File class, such as File.documentsDirectory.

Common directory locations

Platform	Directory type	Typical file system location
Android	Application	/data/data/
	Application-storage	/data/data/air.applicationID/filename/Local Store
	Cache	/data/data/applicationID/cache
	Desktop	/mnt/sdcard
	Documents	/mnt/sdcard
	Temporary	/data/data/applicationID/cache/FlashTmp.randomString
	User	/mnt/sdcard
iOS	Application	/var/mobile/Applications/uid/filename.app
	Application-storage	/var/mobile/Applications/uid/Library/Application Support/applicationID/Local Store
	Cache	/var/mobile/Applications/uid/Library/Caches
	Desktop	not accessible
	Documents	/var/mobile/Applications/uid/Documents
	Temporary	/private/var/mobile/Applications/uid/tmp/FlashTmpNNN
	User	not accessible
Linux	Application	/opt/filename/share
	Application-storage	/home/userName/.appdata/applicationID/Local Store
	Desktop	/home/userName/Desktop
	Documents	/home/userName/Documents
	Temporary	/tmp/FlashTmp.randomString
	User	/home/userName
Mac	Application	/Applications/filename.app/Contents/Resources
	Application-storage	/Users/ userName/Library/Preferences/ applicationid/Local Store (AIR 3.2 and earlier) path /Library/Application Support/ applicationid /Local Store (AIR 3.3 and later), where path is is either /Users/userName /Library/Containers/bundle-id /Data (sandboxed environment) or /Users/userName (when running outside a sandboxed environment)
	Cache	/Users/userName/Library/Caches
	Desktop	/Users/ userName/Desktop
	Documents	/Users/ userName/Documents
	Temporary	/private/var/folders/JY/randomString/TemporaryItems/FlashTmp
	User	/Users/ userName
Windows	Application	C:\Program Files\filename
	Application-storage	C:\Documents and settings\userName\ApplicationData\applicationID\Local Store
	Cache	C:\Documents and settings\userName\Local Settings\Temp
	Desktop	C:\Documents and settings\userName\Desktop
	Documents	C:\Documents and settings\userName\My Documents
	Temporary	C:\Documents and settings\userName\Local Settings\Temp\randomString.tmp
	User	C:\Documents and settings\userName

Common directory locations

The actual native paths for these directories vary based on the operating system and computer configuration. The paths shown in this table are typical examples. You should always use the appropriate static File class properties to refer to these directories so that your application works correctly on any platform. In an actual AIR application, the values for applicationID and filename shown in the table are taken from the application descriptor. If you specify a publisher ID in the application descriptor, then the publisher ID is appended to the application ID in these paths. The value for userName is the account name of the installing user.

Pointing a File object to a directory

There are different ways to set a File object to point to a directory.

Pointing to the user's home directory

You can point a File object to the user's home directory. The following code sets a File object to point to an AIR Test subdirectory of the home directory:

var file:File = File.userDirectory.resolvePath("AIR Test");

Pointing to the user's documents directory

You can point a File object to the user's documents directory. The following code sets a File object to point to an AIR Test subdirectory of the documents directory:

var file:File = File.documentsDirectory.resolvePath("AIR Test");

Pointing to the desktop directory

You can point a File object to the desktop. The following code sets a File object to point to an AIR Test subdirectory of the desktop:

var file:File = File.desktopDirectory.resolvePath("AIR Test");

Pointing to the application storage directory

You can point a File object to the application storage directory. For every AIR application, there is a unique associated path that defines the application storage directory. This directory is unique to each application and user. You can use this directory to store user-specific, application-specific data (such as user data or preferences files). For example, the following code points a File object to a preferences file, prefs.xml, contained in the application storage directory:

var file:File = File.applicationStorageDirectory;
file = file.resolvePath("prefs.xml");

The application storage directory location is typically based on the user name and the application ID. The following file system locations are given here to help you debug your application. You should always use the File.applicationStorage property or app-storage: URI scheme to resolve files in this directory:

• On Mac OS — varies by AIR version:

  AIR 3.2 and earlier : /Users/user name/Library/Preferences/ applicationID /Local Store/

  AIR 3.3 and later : path/Library/Application Support/applicationID /Local Store, where path is either /Users/ username/Library/Containers/bundle-id /Data (sandboxed environment) or /Users/ username (when running outside a sandboxed environment)

  For example (AIR 3.2):

  /Users/babbage/Library/Preferences/com.example.TestApp/Local Store

• On Windows—In the documents and Settings directory, in:

  C:\Documents and Settings\user name \Application Data\ applicationID \Local Store\

  For example:

  C:\Documents and Settings\babbage\Application Data\com.example.TestApp\Local Store

• On Linux—In:

  /home/ user name /.appdata/ applicationID /Local Store/

  For example:

  /home/babbage/.appdata/com.example.TestApp/Local Store

• On Android—In:

  /data/data/ androidPackageID / applicationID/Local Store

  For example:

  /data/data/air.com.example.TestApp/com.example.TestApp/Local Store

Note: If an application has a publisher ID, then the publisher ID is also used as part of the path to the application storage directory.

The URL (and url property) for a File object created with File.applicationStorageDirectory uses the app-storage URL scheme (see Supported AIR URL schemes), as in the following:

var dir:File = File.applicationStorageDirectory;
dir = dir.resolvePath("preferences");
trace(dir.url); // app-storage:/preferences

Pointing to the application directory

You can point a File object to the directory in which the application was installed, known as the application directory. You can reference this directory using the File.applicationDirectory property. You can use this directory to examine the application descriptor file or other resources installed with the application. For example, the following code points a File object to a directory named images in the application directory:

var dir:File = File.applicationDirectory;
dir = dir.resolvePath("images");

The URL (and url property) for a File object created with File.applicationDirectory uses the app URL scheme (see Supported AIR URL schemes), as in the following:

var dir:File = File.applicationDirectory;
dir = dir.resolvePath("images");
trace(dir.url); // app:/images

Note: On Android, the files in the application package are not accessible via the nativePath. The nativePath property is an empty string. Always use the URL to access files in the application directory rather than a native path.

Pointing to the cache directory

You can point a File object to the operating system's temporary or cache directory using the File.cacheDirectory property. This directory contains temporary files that are not required for the application to run and will not cause problems or data loss for the user if they are deleted.

In most operating systems the cache directory is a temporary directory. On iOS, the cache directory corresponds to the application library's Caches directory. Files in this directory are not backed up to online storage, and can potentially be deleted by the operating system if the device's available storage space is too low. For more information, see Controlling file backup and caching.

Pointing to the file system root

The File.getRootDirectories() method lists all root volumes, such as C: and mounted volumes, on a Windows computer. On Mac OS and Linux, this method always returns the unique root directory for the machine (the "/" directory). The StorageVolumeInfo.getStorageVolumes() method provides more detailed information on mounted storage volumes (see Working with storage volumes).

Note: The root of the file system is not readable on Android. A File object referencing the directory with the native path, "/", is returned, but the properties of that object do not have accurate values. For example, spaceAvailable is always 0.

Pointing to an explicit directory

You can point the File object to an explicit directory by setting the nativePath property of the File object, as in the following example (on Windows):

var file:File = new File();
file.nativePath = "C:\\AIR Test";

Important: Pointing to an explicit path this way can lead to code that does not work across platforms. For example, the previous example only works on Windows. You can use the static properties of the File object, such as File.applicationStorageDirectory, to locate a directory that works cross-platform. Then use the resolvePath() method (see the next section) to navigate to a relative path.

Navigating to relative paths

You can use the resolvePath() method to obtain a path relative to another given path. For example, the following code sets a File object to point to an "AIR Test" subdirectory of the user's home directory:

var file:File = File.userDirectory;
file = file.resolvePath("AIR Test");

You can also use the url property of a File object to point it to a directory based on a URL string, as in the following:

var urlStr:String = "file:///C:/AIR Test/";
var file:File = new File()
file.url = urlStr;

For more information, see Modifying File paths.

Letting the user browse to select a directory

The File class includes the browseForDirectory() method, which presents a system dialog box in which the user can select a directory to assign to the object. The browseForDirectory() method is asynchronous. The File object dispatches a select event if the user selects a directory and clicks the Open button, or it dispatches a cancel event if the user clicks the Cancel button.

For example, the following code lets the user select a directory and outputs the directory path upon selection:

var file:File = new File();
file.addEventListener(Event.SELECT, dirSelected);
file.browseForDirectory("Select a directory");
function dirSelected(e:Event):void {
    trace(file.nativePath);
}

Note: On Android, the browseForDirectory() method is not supported. Calling this method has no effect; a cancel event is dispatched immediately. To allow users to select a directory, use a custom, application-defined dialog, instead.

Pointing to the directory from which the application was invoked

You can get the directory location from which an application is invoked, by checking the currentDirectory property of the InvokeEvent object dispatched when the application is invoked. For details, see Capturing command line arguments.

Pointing a File object to a file

There are different ways to set the file to which a File object points.

Pointing to an explicit file path

Important: Pointing to an explicit path can lead to code that does not work across platforms. For example, the path C:/foo.txt only works on Windows. You can use the static properties of the File object, such as File.applicationStorageDirectory, to locate a directory that works cross-platform. Then use the resolvePath() method (see Modifying File paths) to navigate to a relative path.

You can use the url property of a File object to point it to a file or directory based on a URL string, as in the following:

var urlStr:String = "file:///C:/AIR Test/test.txt";
var file:File = new File()
file.url = urlStr;

You can also pass the URL to the File() constructor function, as in the following:

var urlStr:String = "file:///C:/AIR Test/test.txt";
var file:File = new File(urlStr);

The url property always returns the URI-encoded version of the URL (for example, blank spaces are replaced with "%20):

file.url = "file:///c:/AIR Test";
trace(file.url); // file:///c:/AIR%20Test

You can also use the nativePath property of a File object to set an explicit path. For example, the following code, when run on a Windows computer, sets a File object to the test.txt file in the AIR Test subdirectory of the C: drive:

var file:File = new File();
file.nativePath = "C:/AIR Test/test.txt";

You can also pass this path to the File() constructor function, as in the following:

var file:File = new File("C:/AIR Test/test.txt");

Use the forward slash (/) character as the path delimiter for the nativePath property. On Windows, you can also use the backslash (\ character, but doing so leads to applications that do not work across platforms.

For more information, see Modifying File paths.

Enumerating files in a directory

You can use the getDirectoryListing() method of a File object to get an array of File objects pointing to files and subdirectories at the root level of a directory. For more information, see Enumerating directories.

Letting the user browse to select a file

The File class includes the following methods that present a system dialog box in which the user can select a file to assign to the object:

• browseForOpen()

• browseForSave()

• browseForOpenMultiple()

These methods are each asynchronous. The browseForOpen() and browseForSave() methods dispatch the select event when the user selects a file (or a target path, in the case of browseForSave()). With the browseForOpen() and browseForSave() methods, upon selection the target File object points to the selected files. The browseForOpenMultiple () method dispatches a selectMultiple event when the user selects files. The selectMultiple event is of type FileListEvent, which has a files property that is an array of File objects (pointing to the selected files).

For example, the following code presents the user with an "Open" dialog box in which the user can select a file:

var fileToOpen:File = File.documentsDirectory;
selectTextFile(fileToOpen);

function selectTextFile(root:File):void
{
    var txtFilter:FileFilter = new FileFilter("Text", "*.as;*.css;*.html;*.txt;*.xml");
    root.browseForOpen("Open", [txtFilter]);
    root.addEventListener(Event.SELECT, fileSelected);
}

function fileSelected(event:Event):void
{
    trace(fileToOpen.nativePath);
}

If the application has another browser dialog box open when you call a browse method, the runtime throws an Error exception.

Note: On Android, only image, video, and audio files can be selected with the browseForOpen() and browseForOpenMultiple() methods. The browseForSave() dialog also displays only media files even though the user can enter an arbitrary filename. For opening and saving non-media files, you should consider using custom dialogs instead of these methods.

Modifying File paths

You can also modify the path of an existing File object by calling the resolvePath() method or by modifying the nativePath or url property of the object, as in the following examples (on Windows):

var file1:File = File.documentsDirectory;
file1 = file1.resolvePath("AIR Test");
trace(file1.nativePath); // C:\Documents and Settings\userName\My Documents\AIR Test
var file2:File = File.documentsDirectory;
file2 = file2.resolvePath("..");
trace(file2.nativePath); // C:\Documents and Settings\userName
var file3:File = File.documentsDirectory;
file3.nativePath += "/subdirectory";
trace(file3.nativePath); // C:\Documents and Settings\userName\My Documents\subdirectory
var file4:File = new File();
file4.url = "file:///c:/AIR Test/test.txt";
trace(file4.nativePath); // C:\AIR Test\test.txt

When using the nativePath property, use the forward slash (/) character as the directory separator character. On Windows, you can use the backslash (\ character as well, but you should not do so, as it leads to code that does not work cross-platform.

Supported AIR URL schemes

In AIR, you can use any of the following URL schemes in defining the url property of a File object:

URL scheme	Description
file	Use to specify a path relative to the root of the file system. For example: file:///c:/AIR Test/test.txt The URL standard specifies that a file URL takes the formfile://<host>/<path>. As a special case,<host> can be the empty string, which is interpreted as "the machine from which the URL is being interpreted." For this reason, file URLs often have three slashes (///).
app	Use to specify a path relative to the root directory of the installed application (the directory that contains the application.xml file for the installed application). For example, the following path points to an images subdirectory of the directory of the installed application: app:/images
app-storage	Use to specify a path relative to the application store directory. For each installed application, AIR defines a unique application store directory, which is a useful place to store data specific to that application. For example, the following path points to a prefs.xml file in a settings subdirectory of the application store directory: app-storage:/settings/prefs.xml

Controlling file backup and caching

Certain operating systems, most notably iOS and Mac OS X, provide users the ability to automatically back up application files to a remote storage. In addition, on iOS there are restrictions on whether files can be backed up and also where files of different purposes can be stored.

The following summarize how to comply with Apple's guidelines for file backup and storage. For further information see the next sections.

• To specify that a file does not need to be backed up and (iOS only) can be deleted by the operating system if device storage space runs low, save the file in the cache directory ( File.cacheDirectory). This is the preferred storage location on iOS and should be used for most files that can be regenerated or re-downloaded.

• To specify that a file does not need to be backed up, but should not be deleted by the operating system, save the file in one of the application library directories such as the application storage directory ( File.applicationStorageDirectory) or the documents directory ( File.documentsDirectory). Set the File object's preventBackup property to true. This is required by Apple for content that can be regenerated or downloaded again, but which is required for proper functioning of your application during offline use.

Specifying files for backup

In order to save backup space and reduce network bandwidth use, Apple's guidelines for iOS and Mac applications specify that only files that contain user-entered data or data that otherwise can't be regenerated or re-downloaded should be designated for backup.

By default all files in the application library folders are backed up. On Mac OS X this is the application storage directory. On iOS, this includes the application storage directory, the application directory, the desktop directory, documents directory, and user directory (because those directories are mapped to application library folders on iOS). Consequently, any files in those directories are backed up to server storage by default.

If you are saving a file in one of those locations that can be re-created by your application, you should flag the file so the operating system knows not to back it up. To indicate that a file should not be backed up, set the File object's preventBackup property to true.

Note that on iOS, for a file in any of the application library folders, even if the file's preventBackup property is set to true the file is flagged as a persistent file that the operating system shouldn't delete.

Controlling file caching and deletion

Apple's guidelines for iOS applications specify that as much as possible, content that can be regenerated should be made available to the operating system to delete in case the device runs low on storage space.

On iOS, files in the application library folders (such as the application storage directory or the documents directory) are flagged as permanent and are not deleted by the operating system.

Save files that can be regenerated by the application and are safe to delete in case of low storage space in the application cache directory. You access the cache directory using the File.cacheDirectory static property.

On iOS the cache directory corresponds to the application's cache directory (\<Application Home>/Library/Caches). On other operating systems, this directory is mapped to a comparable directory. For example, on Mac OS X it also maps to the Caches directory in the application library. On Android the cache directory maps to the application's cache directory. On Windows, the cache directory maps to the operating system temp directory. On both Android and Windows, this is the same directory that is accessed by a call to the File class's createTempDirectory() and createTempFile() methods.

Finding the relative path between two files

You can use the getRelativePath() method to find the relative path between two files:

var file1:File = File.documentsDirectory.resolvePath("AIR Test");
var file2:File = File.documentsDirectory
file2 = file2.resolvePath("AIR Test/bob/test.txt");

trace(file1.getRelativePath(file2)); // bob/test.txt

The second parameter of the getRelativePath() method, the useDotDot parameter, allows for .. syntax to be returned in results, to indicate parent directories:

var file1:File = File.documentsDirectory;
file1 = file1.resolvePath("AIR Test");
var file2:File = File.documentsDirectory;
file2 = file2.resolvePath("AIR Test/bob/test.txt");
var file3:File = File.documentsDirectory;
file3 = file3.resolvePath("AIR Test/susan/test.txt");

trace(file2.getRelativePath(file1, true)); // ../..
trace(file3.getRelativePath(file2, true)); // ../../bob/test.txt

Obtaining canonical versions of file names

File and path names are not case sensitive on Windows and Mac OS. In the following, two File objects point to the same file:

File.documentsDirectory.resolvePath("test.txt");
File.documentsDirectory.resolvePath("TeSt.TxT");

However, documents and directory names do include capitalization. For example, the following assumes that there is a folder named AIR Test in the documents directory, as in the following examples:

var file:File = File.documentsDirectory.resolvePath("AIR test");
trace(file.nativePath); // ... AIR test
file.canonicalize();
trace(file.nativePath); // ... AIR Test

The canonicalize() method converts the nativePath object to use the correct capitalization for the file or directory name. On case sensitive file systems (such as Linux), when multiple files exists with names differing only in case, the canonicalize() method adjusts the path to match the first file found (in an order determined by the file system).

You can also use the canonicalize() method to convert short file names ("8.3" names) to long file names on Windows, as in the following examples:

var path:File = new File();
path.nativePath = "C:\\AIR~1";
path.canonicalize();
trace(path.nativePath); // C:\AIR Test

Working with packages and symbolic links

Various operating systems support package files and symbolic link files:

Packages —On Mac OS, directories can be designated as packages and show up in the Mac OS Finder as a single file rather than as a directory.

Symbolic links —Mac OS, Linux, and Windows Vista support symbolic links. Symbolic links allow a file to point to another file or directory on disk. Although similar, symbolic links are not the same as aliases. An alias is always reported as a file (rather than a directory), and reading or writing to an alias or shortcut never affects the original file or directory that it points to. On the other hand, a symbolic link behaves exactly like the file or directory it points to. It can be reported as a file or a directory, and reading or writing to a symbolic link affects the file or directory that it points to, not the symbolic link itself. Additionally, on Windows the isSymbolicLink property for a File object referencing a junction point (used in the NTFS file system) is set to true.

The File class includes the isPackage and isSymbolicLink properties for checking if a File object references a package or symbolic link.

The following code iterates through the user's desktop directory, listing subdirectories that are not packages:

var desktopNodes:Array = File.desktopDirectory.getDirectoryListing();
for (var i:uint = 0; i < desktopNodes.length; i++)
{
    if (desktopNodes[i].isDirectory && !desktopNodes[i].isPackage)
    {
        trace(desktopNodes[i].name);
    }
}

The following code iterates through the user's desktop directory, listing files and directories that are not symbolic links:

var desktopNodes:Array = File.desktopDirectory.getDirectoryListing();
for (var i:uint = 0; i < desktopNodes.length; i++)
{
    if (!desktopNodes[i].isSymbolicLink)
    {
        trace(desktopNodes[i].name);
    }
}

The canonicalize() method changes the path of a symbolic link to point to the file or directory to which the link refers. The following code iterates through the user's desktop directory, and reports the paths referenced by files that are symbolic links:

var desktopNodes:Array = File.desktopDirectory.getDirectoryListing();
for (var i:uint = 0; i < desktopNodes.length; i++)
{
    if (desktopNodes[i].isSymbolicLink)
    {
        var linkNode:File = desktopNodes[i] as File;
        linkNode.canonicalize();
        trace(linkNode.nativePath);
    }
}

Determining space available on a volume

The spaceAvailable property of a File object is the space available for use at the File location, in bytes. For example, the following code checks the space available in the application storage directory:

trace(File.applicationStorageDirectory.spaceAvailable);

If the File object references a directory, the spaceAvailable property indicates the space in the directory that files can use. If the File object references a file, the spaceAvailable property indicates the space into which the file could grow. If the file location does not exist, the spaceAvailable property is set to 0. If the File object references a symbolic link, the spaceAvailable property is set to space available at the location the symbolic link points to.

Typically the space available for a directory or file is the same as the space available on the volume containing the directory or file. However, space available can take into account quotas and per-directory limits.

Adding a file or directory to a volume generally requires more space than the actual size of the file or the size of the contents of the directory. For example, the operating system may require more space to store index information. Or the disk sectors required may use additional space. Also, available space changes dynamically. So, you cannot expect to allocate all of the reported space for file storage. For information on writing to the file system, see Reading and writing files.

The StorageVolumeInfo.getStorageVolumes() method provides more detailed information on mounted storage volumes (see Working with storage volumes).

Opening files with the default system application

In AIR 2, you can open a file using the application registered by the operating system to open it. For example, an AIR application can open a DOC file with the application registered to open it. Use the openWithDefaultApplication() method of a File object to open the file. For example, the following code opens a file named test.doc on the user's desktop and opens it with the default application for DOC files:

var file:File = File.deskopDirectory;
file = file.resolvePath("test.doc");
file.openWithDefaultApplication();

Note: On Linux, the file's MIME type, not the filename extension, determines the default application for a file.

The following code lets the user navigate to an mp3 file and open it in the default application for playing mp3 files:

var file:File = File.documentsDirectory;
var mp3Filter:FileFilter = new FileFilter("MP3 Files", "*.mp3");
file.browseForOpen("Open", [mp3Filter]);
file.addEventListener(Event.SELECT, fileSelected);

function fileSelected(e:Event):void
{
    file.openWithDefaultApplication();
}

You cannot use the openWithDefaultApplication() method with files located in the application directory.

AIR prevents you from using the openWithDefaultApplication() method to open certain files. On Windows, AIR prevents you from opening files that have certain filetypes, such as EXE or BAT. On Mac OS and Linux, AIR prevents you from opening files that will launch in certain application. (These include Terminal and AppletLauncher on Mac OS; and csh, bash, or ruby on Linux.) Attempting to open one of these files using the openWithDefaultApplication() method results in an exception. For a complete list of prevented filetypes, see the language reference entry for the File.openWithDefaultApplication() method.

Note: This limitation does not exist for an AIR application installed using a native installer (an extended desktop application).