static findTransmuxer(mimeType, contentType) {
    const TransmuxerEngine = shaka.transmuxer.TransmuxerEngine;
    const normalizedMimetype = TransmuxerEngine.normalizeMimeType_(mimeType);
    const priorities = [
      TransmuxerEngine.PluginPriority.APPLICATION,
      TransmuxerEngine.PluginPriority.PREFERRED,
      TransmuxerEngine.PluginPriority.PREFERRED_SECONDARY,
      TransmuxerEngine.PluginPriority.FALLBACK,
    ];
    for (const priority of priorities) {
      const key = normalizedMimetype + '-' + priority;
      const object = TransmuxerEngine.transmuxerMap_.get(key);
      if (object) {
        const transmuxer = object.plugin();
        const isSupported = transmuxer.isSupported(mimeType, contentType);
        transmuxer.destroy();
        if (isSupported) {
          return object.plugin;
        }
      }
    }
    return null;
  }
@param {string} mimeType
@param {string=} contentType
@return {?shaka.extern.TransmuxerPlugin}
@export