The SWF cannot mail at all.
You have to send the attachment to a server, which does the mailing.
How this is done depends on the hosting you are using.
If you look into our little tutorial, you can see, that the serverside script is sending the message.
For Attachment you have to create a valid envelope with multiparted content.
A different, and very efficient way, is store the file at the server, and send a link to the receipient.