isPreloadAllowed_(interstitial) {
    const interstitialMimeType = interstitial.mimeType;
    if (!interstitialMimeType) {
      return true;
    }
    return !interstitialMimeType.startsWith('image/') &&
        interstitialMimeType !== 'text/html';
  }
@param {!shaka.extern.AdInterstitial} interstitial
@return {boolean}
@private