static getExtension_(uri) {
    const uriObj = new goog.Uri(uri);
    const uriPieces = uriObj.getPath().split('/');
    const uriFilename = uriPieces.pop();
    const filenamePieces = uriFilename.split('.');

    // Only one piece means there is no extension.
    if (filenamePieces.length == 1) {
      return '';
    }

    return filenamePieces.pop().toLowerCase();
  }
@param {string} uri
@return {string}
@private