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

def _extract_links_from_lecture_assets(self, asset_ids):
        """
        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
        """
        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