loadFiles(callback) {
    var promises = [];
    // Load file content
    this.files.forEach((file, i) => {
      file.getContent((success, error) => {
        if (error) callback(success, error);
        promises.push(i);
        file.parseContent();
        // Execute after all content is loaded
        if (this.files.length == promises.length) {
          callback(success, error);
        }
      });
    });
  }
Load files and get file content.
@method
@async
@param {function} callback - Callback function