As of this time, it is possible to attach an image to a scheduled message, but no other types of automatic messages, and for rules sending messages until the day of the check-out. Other file types are not supported. Only one image at a time can be attached.
An alternative solution to sending multiple images or other file types to your guests would be:
- Upload the file(s) to a cloud storage such as Dropbox or Google Drive.
- Set the file sharing permissions to Anyone with the link.
- Copy the file URL.
- (Optional) Trim or customize the URL using a URL shortener such as bit.ly.
- Paste the final URL into your automatic message template.