static isSupported(mimeType, contentType) {
    const TransmuxerEngine = shaka.transmuxer.TransmuxerEngine;
    const transmuxerPlugin = TransmuxerEngine.findTransmuxer(mimeType);
    if (!transmuxerPlugin) {
      return false;
    }
    return true;
  }
Check if the mime type and the content type is supported.
@param {string} mimeType
@param {string=} contentType
@return {boolean}