I think currently the most important part of emails are attachments.
We leave in 21 st century, and we sending many photos, mp3, project or any other files by e-mail.
I have to ask, are you concern to allow multiple file upload at this same time, the best idea will be in flash.
there you can see demo
SWFUpload Demos - Application Demo
for me, this idea gets 10 / 10 score.
Allow to chose 5 mp3, 2 docs in time, just select using ctrl.
No clicking, "add more attachment".
One click, and everything will be uploaded.
For people what won't use flash, you can make option in preferences "use flash upload" or default.