code: function _getExtensionRegEx(ext=settings.get('extensions')) {
    let _ext = '(?:' + makeArray(ext).join('|') + ')';
    return new RegExp(_ext + '$');
  }
docs: Get a regular expression for the given selection of file extensions, which
  will then be able to match file paths, which have those extensions.
  
  @private
  @param {Array|string} [ext=config.get('extensions')]     The extension(s).
  @returns {RegExp}                                        File path matcher.