adding support for the “m4a” format

Author: zhaosiyang

lib/src/linux/kdialog_handler.dart

@@ -56,7 +56,7 @@ class KDialogHandler implements DialogHandler {
       case FileType.any:
         return '';
       case FileType.audio:
-        return 'Audio File (${DialogHandler.toCaseInsensitive("*.aac *.midi *.mp3 *.ogg *.wav")})';
+        return 'Audio File (${DialogHandler.toCaseInsensitive("*.aac *.midi *.mp3 *.ogg *.wav *.m4a")})';
       case FileType.custom:
         return '${allowedExtensions!.map((ext) => ext.toUpperCase()).join(' File, ')} File (${DialogHandler.toCaseInsensitive("*.${allowedExtensions.join(' *.')}")})';
       case FileType.image:

lib/src/linux/qarma_and_zenity_handler.dart

@@ -48,7 +48,7 @@ class QarmaAndZenityHandler implements DialogHandler {
       case FileType.any:
         return '';
       case FileType.audio:
-        return "Audio Files | ${DialogHandler.toCaseInsensitive('*.aac *.midi *.mp3 *.ogg *.wav')}";
+        return "Audio Files | ${DialogHandler.toCaseInsensitive('*.aac *.midi *.mp3 *.ogg *.wav *.m4a')}";
       case FileType.custom:
         return "Custom Files | ${DialogHandler.toCaseInsensitive('*.${allowedExtensions!.join(' *.')}')}";
       case FileType.image:

lib/src/windows/file_picker_windows.dart

@@ -218,7 +218,7 @@ class FilePickerWindows extends FilePicker {
       case FileType.any:
         return 'All Files (*.*)\x00*.*\x00\x00';
       case FileType.audio:
-        return 'Audios (*.aac,*.midi,*.mp3,*.ogg,*.wav)\x00*.aac;*.midi;*.mp3;*.ogg;*.wav\x00\x00';
+        return 'Audios (*.aac,*.midi,*.mp3,*.ogg,*.wav,*.m4a)\x00*.aac;*.midi;*.mp3;*.ogg;*.wav;*.m4a\x00\x00';
       case FileType.custom:
         return 'Files (*.${allowedExtensions!.join(',*.')})\x00*.${allowedExtensions.join(';*.')}\x00\x00';
       case FileType.image:

test/file_picker_windows_test.dart

@@ -23,7 +23,7 @@ void main() {
       expect(
         picker.fileTypeToFileFilter(FileType.audio, null),
         equals(
-            'Audios (*.aac,*.midi,*.mp3,*.ogg,*.wav)\x00*.aac;*.midi;*.mp3;*.ogg;*.wav\x00\x00'),
+            'Audios (*.aac,*.midi,*.mp3,*.ogg,*.wav,*.m4a)\x00*.aac;*.midi;*.mp3;*.ogg;*.wav;*.m4a\x00\x00'),
       );
 
       expect(