code: def ext(self):
          
          if self._filename:
              return os.path.splitext(self._filename)[1].lstrip('.')
          return {
              CT.ASF:        'asf',
              CT.AVI:        'avi',
              CT.MOV:        'mov',
              CT.MP4:        'mp4',
              CT.MPG:        'mpg',
              CT.MS_VIDEO:   'avi',
              CT.SWF:        'swf',
              CT.WMV:        'wmv',
              CT.X_MS_VIDEO: 'avi',
          }.get(self._mime_type, 'vid')
docs: Return the file extension for this video, e.g. 'mp4'.
  
          The extension is that from the actual filename if known. Otherwise
          it is the lowercase canonical extension for the video's MIME type.
          'vid' is used if the MIME type is 'video/unknown'.