Merge pull request #1622 from rbrito/fix-extension

extractor: youtube: Set extension of AAC audio formats to m4a.

diff --combined youtube_dl/extractor/youtube.py
index a88cba2b4d74d2f57d0b781ab8e608e693c1caa2,96ead331042ad918de9db6bc5a2612192d122659..7a7bbe26511436e8fae54f6f07ed5a3f741d48ba
--- 1/youtube_dl/extractor/youtube.py
--- 2/youtube_dl/extractor/youtube.py
+++ b/youtube_dl/extractor/youtube.py
@@@ -236,11 -236,13 +236,13 @@@ class YoutubeIE(YoutubeBaseInfoExtracto
          '136': 'mp4',
          '137': 'mp4',
          '138': 'mp4',
-         '139': 'mp4',
-         '140': 'mp4',
-         '141': 'mp4',
          '160': 'mp4',
  
+         # Dash mp4 audio
+         '139': 'm4a',
+         '140': 'm4a',
+         '141': 'm4a',
+ 
          # Dash webm
          '171': 'webm',
          '172': 'webm',
@@@ -1150,7 -1152,7 +1152,7 @@@
              list_page = self._download_webpage(list_url, video_id)
              caption_list = xml.etree.ElementTree.fromstring(list_page.encode('utf-8'))
              original_lang_node = caption_list.find('track')
 -            if not original_lang_node or original_lang_node.attrib.get('kind') != 'asr' :
 +            if original_lang_node is None or original_lang_node.attrib.get('kind') != 'asr' :
                  self._downloader.report_warning(u'Video doesn\'t have automatic captions')
                  return {}
              original_lang = original_lang_node.attrib['lang_code']