async getTextMimetype_(uri) {
    let mimeType;
    try {
      goog.asserts.assert(
          this.networkingEngine_, 'Need networking engine.');
      mimeType = await shaka.net.NetworkingUtils.getMimeType(uri,
          this.networkingEngine_,
          this.config_.streaming.retryParameters);
    } catch (error) {}

    if (mimeType) {
      return mimeType;
    }

    shaka.log.error(
        'The mimeType has not been provided and it could not be deduced ' +
        'from its uri.');
    throw new shaka.util.Error(
        shaka.util.Error.Severity.RECOVERABLE,
        shaka.util.Error.Category.TEXT,
        shaka.util.Error.Code.TEXT_COULD_NOT_GUESS_MIME_TYPE,
        uri);
  }
@param {string} uri
@return {!Promise<string>}
@private