Welcome back to the community, @Steve_Ruqus! :grinning_face_with_big_eyes: Unfortunately this isn’t possible. The
mailto protocol isn’t designed to pass files as URL properties. Only the message body and header fields are supported.
There might be a way to do it using a script—which would still give you the button-level control that you want—but I haven’t delved into accessing Google’s API at that level, so I couldn’t offer more specific guidance.
Another option would be to insert a link to the file into the message body using the URL of the attachment, which can be extracted from the attachment field using a formula.