It is possible to successfully setup sending of attachments from a process, but it requires triggers to get it to work. Basically, there is a field in tps_user_message which is called tps_attachment which is an xml data type. There is a further table called tps_attachment_data which contains the guid and a hexadecimal field which records the attachment data. If you have an attachment stored in tps_attachment_data then you can generate the XML required using a trigger and populate the tps_attachment field in tps_user_message.
Obviously it does require scripting which is not supported...
This should help to get on track.