code: func (s *AppsService) CreateAttachment(ctx context.Context, contentReferenceID int64, title, body string) (*Attachment, *Response, error) {
  	u := fmt.Sprintf("content_references/%v/attachments", contentReferenceID)
  	payload := &Attachment{Title: String(title), Body: String(body)}
  	req, err := s.client.NewRequest("POST", u, payload)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	// TODO: remove custom Accept headers when APIs fully launch.
  	req.Header.Set("Accept", mediaTypeReactionsPreview)
  
  	m := &Attachment{}
  	resp, err := s.client.Do(ctx, req, m)
  	if err != nil {
  		return nil, resp, err
  	}
  
  	return m, resp, nil
  }
docs: // Create a new attachment on user comment containing a url.
  //
  // GitHub API docs: https://developer.github.com/v3/apps/#create-a-content-attachment