Rated 5 out of 5 stars
One thing to note: do not delete the attachment from the message. Doing so removes the file from Dropbox.
I made this mistake. Thought the problem was Dropbox for Filelink. Just don't delete the files from the attachment pane. The files will show "as if attached" including their original file size. When the message is sent, the files will continue to show as attachments but with 0-bytes file size.