extractor: youtube: Fix extension of dash formats.

While we are at it, separate the audio formats from the video formats.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>

youtube_dl/extractor/youtube.py

@@ -236,10 +236,12 @@ class YoutubeIE(YoutubeBaseInfoExtractor, SubtitlesInfoExtractor):
         '136': 'mp4',
         '137': 'mp4',
         '138': 'mp4',
-        '139': 'mp4',
+        '160': 'mp4',
+
+        # Dash mp4 audio
+        '139': 'm4a',
         '140': 'm4a',
         '141': 'm4a',
-        '160': 'm4a',
 
         # Dash webm
         '171': 'webm',