[youtube] Use mp4 as extension for format 38 (Fixes #892)

youtube_dl/extractor/youtube.py

@@ -58,7 +58,7 @@ class YoutubeIE(InfoExtractor):
         '18': 'mp4',
         '22': 'mp4',
         '37': 'mp4',
-        '38': 'video', # You actually don't know if this will be MOV, AVI or whatever
+        '38': 'mp4',
         '43': 'webm',
         '44': 'webm',
         '45': 'webm',