Adding extension for attachments

you mean body part, right? and "file contents" is vague. "SHOULD treat the `value` property of the associated `data` object as a filename and fill the body with the contents of that file" or something more. 

honestly, i thought we were going to avoid this prose by just referring to the RFC2388 and I-D. why is it important to add this in here (just remind me, OK?)