addExtraFeaturesToMimeType_(mimeType) {
    const extraFeatures = this.config_.addExtraFeaturesToSourceBuffer(mimeType);
    const extendedType = mimeType + extraFeatures;
    shaka.log.debug('Using full mime type', extendedType);

    return extendedType;
  }
@param {string} mimeType
@return {string}
@private