Direct mails are physical post emails which you will be sending in an envelope using a third party vendor. Maybe I'm not seeing the full picture but attachment in this scenario doesn't look to be a good idea.
My goal was to create a new template for Direct Mail. When one uses the Bulk Delivery option, campaign provides you ability to add a .odt file to the delivery and process the personalization rule in that file (Trying to apply the concept of Attachments in the direct mail delivery).
Hence, I asked what's the purpose of "account"?
But, as i understand from your comment, we are not suppose to use that "Bulk" delivery option for Direct Mail channel. Is this assumption correct?