The property requires a ExtensionRestrictions enum assignment.
No filter is applied to the FileExplorer control instance. All files are shown.
The extension filter is applied based on the declared extensions in
WMAppManifest.xml. This is only useful if the StorageTarget is set to
IsolatedStorage. If StorageTarget is set to
ExternalStorage, the manifest-based extension filter is applied automatically.
A custom List<string> declared through Extensions is applied as a filter. If
StorageTarget is set to ExternalStorage, the custom filter is applied in addition to the automatic manifest-based one.