getContent(callback) {
    get(this.url, (success, error) => {
      if (error) callback(success, error);
      this.content = success;
      // check if the response returns a string instead
      // of an response object
      if (typeof this.content === 'string') {
        callback(success, error);
      }
    });
  }
Get file content.
@method
@async
@param {function} callback - Callback function.
@description
Get the file's HTML content and set the file object html
attribute to the file content.