guessMimeTypeFallback_(contentType) {
    if (contentType == shaka.util.ManifestParserUtils.ContentType.TEXT) {
      // If there was no codecs string and no content-type, assume HLS text
      // streams are WebVTT.
      return 'text/vtt';
    }
    // If the HLS content is lacking in both MIME type metadata and
    // segment file extensions, we fall back to assuming it's MP4.
    const map =
        shaka.hls.HlsParser.EXTENSION_MAP_BY_CONTENT_TYPE_.get(contentType);
    return map.get('mp4');
  }
Get a fallback mime type for the content. Used if all the better methods
for determining the mime type have failed.

@param {string} contentType
@return {string}
@private