support m4a file extension

Author: VICrazy

flutter_cache_manager/lib/src/web/mime_converter.dart

@@ -54,6 +54,7 @@ const mimeTypes = {
   'audio/aac': '.aac',
   'audio/x-aac': '.aac',
   'audio/midi audio/x-midi': '.midi',
+  'audio/x-m4a': '.m4a',
   'audio/mpeg': '.mp3',
   'audio/ogg': '.oga',
   'audio/opus': '.opus',