[YoutubeDL] mp3 is compatible with mp4

youtube_dl/YoutubeDL.py

@@ -1380,7 +1380,7 @@ class YoutubeDL(object):
                         video_ext, audio_ext = audio.get('ext'), video.get('ext')
                         if video_ext and audio_ext:
                             COMPATIBLE_EXTS = (
-                                ('mp4', 'm4a', 'm4p', 'm4b', 'm4r', 'm4v'),
+                                ('mp3', 'mp4', 'm4a', 'm4p', 'm4b', 'm4r', 'm4v'),
                                 ('webm')
                             )
                             for exts in COMPATIBLE_EXTS: