Apparently we can basically just lie about what MIME type the music file is...

libremsonic/app.py

@@ -444,7 +444,6 @@ class LibremsonicApp(Gtk.Application):
             uri, stream = CacheManager.get_song_filename_or_stream(
                 song,
                 force_stream=self.state.config.always_stream,
-                format='mp3',
             )
 
             self.state.current_song = song

libremsonic/ui/common/players.py

@@ -337,6 +337,7 @@ class ChromecastPlayer(Player):
         cover_art_url = CacheManager.get_cover_art_url(song.id, 1000)
         self.chromecast.media_controller.play_media(
             file_or_url,
+            # Just pretend that whatever we send it is mp3, even if it isn't.
             'audio/mp3',
             current_time=progress,
             title=song.title,