fix: get m4r working

app.py

@@ -54,6 +54,8 @@ def convert_audio(audio_file, output_format, change_bitrate, bitrate):
 
     # Create the ffmpeg command
     command = ['ffmpeg', '-y', '-i', audio_file]
+    if output_format == "m4r":
+        command.extend(['-f', 'mp4'])
     if change_bitrate:
         command.extend(['-b:a', f'{bitrate}k'])
     command.append(output_audio_file)