Uploading the files is easy. Simply go to billing.snapbill.com/files and make sure to use the filename that you want to appear in the email. Be sure to set it to not encrypt, and if you ever want to update the file just choose Replace in the "what to do if file exists" dropdown.
As mentioned before you need to use the @attachment tag, and simply list it at the top. For instance if you want to attach policy.pdf, your file should look like:
@subject:Welcome to SnapBill
@attachment:policy.pdf
== Hi $client->firstname ==
Welcome to ......