Adding attachments to confirmation message

I've found the filter() for adding attachments to a confirmation message from Appointments+.

However, I'm unable to make it work properly. As far as I can tell, the filter takes a single argument (it's an empty string in the appointments.php file), but it's not clear what it expects to have returned.

Never mind... Have to use the same function as I declare in the add_filter() function... *sigh*