Add attachment to FormMail

On [[login to view URL]][1] you can download functionality (called FormMail) which can be used to have an entered form emailed to your email address. I want to use this functionality to have completed registration form sent to me and the person who entered the form. However, I am missing one thing: I want to add a generetad invoice in pdf format as attachment as well. Creating the pdf is built already; the only thing what must be done is adjusting [login to view URL] so the pdf file is sent as attachment. In the form [login to view URL] you will see a hidden field (pdffile) which contains the name of the file to be sent as attachment.