_getAttachments(callback) {
    return callback(this.files
      .filter(file => fs.existsSync(file.path))
      .map((exists, idx) => [exists, this.files[idx]])
      .filter(([exists, _]) => exists)
      .map(([_, file]) => {
        file.content = fs.readFileSync(file.path);
        return createAttachment(file);
      })
    );
  }
Retrieves webhook payload files from the file system (Only applies to non raw payloads)
@private
@param {Function} callback Function which will receive an array, of attachments found, as the sole argument