code: def _extract_links_from_lecture_assets(self, asset_ids):
          
          links = {}
  
          def _add_asset(name, url, destination):
              filename, extension = os.path.splitext(clean_url(name))
              if extension is '':
                  return
  
              extension = clean_filename(
                  extension.lower().strip('.').strip(),
                  self._unrestricted_filenames)
              basename = clean_filename(
                  os.path.basename(filename),
                  self._unrestricted_filenames)
              url = url.strip()
  
              if extension not in destination:
                  destination[extension] = []
              destination[extension].append((url, basename))
  
          for asset_id in asset_ids:
              for asset in self._get_asset_urls(asset_id):
                  _add_asset(asset['name'], asset['url'], links)
  
          return links
docs: Extract links to files of the asset ids.
  
          @param asset_ids: List of asset ids.
          @type asset_ids: [str]
  
          @return: @see CourseraOnDemand._extract_links_from_text