Well there is no access to the file system, that's the biggest part. You can attach multimedia, but only if it's in the camera roll or photo library. AFAIK, you can't access a third party's multimedia library. To attach documents, they'd have to create a documents library somewhere. It sounds easy of course, but who knows how that would affect iOS. Would it interfere with the other apps who keep their information in a folder called documents already (which they all do)? Neither you nor I know how the iOS subsystem works. Probably easier said than done.
Originally Posted by mfm77
The way I would visualize it happening from a UI perspective is similar to how it functions now in beta 1. Instead of choosing select pictures/videos, change it to select item. Takes you to another small sub-menu asking documents or photos/videos and then go from there. Or maybe in the subject line they can add an attachment icon.