I haven't recently been using that module myself, but I have had one other report of this. I did some on-line research, and it appears that recent versions of Mail are responding to the command to send the email, before the attachments have been added. The solution appears to be to insert a delay before sending.
I've uploaded a revised version to my Dropbox account. It checks to see if there are any attachments, and if there are, it inserts a one second delay into the AppleScript before sending. The Dropbox link is