Are you using the custom email transport that is posted to the community? If so, this service has not been enhanced since the attachment feature was provided in the product. We can take a look at it and see if we can release a new version that also allows attachments to be included.
Yes, referring to the custom email transport. Should I assume then that HTML can't be done via the submit e-mail service?
When you create an email you do have the option to view its source (the button is on the right side and looks like a page with the angle brackets on it "<>"). The content generated is HTML, but you do not have 100% control over it as there is some other code that FEB/Leap adds when it sends the email from the Submit activity. The custom service sends exactly the content that you provide it.