There should be no special permissions to upload files. If you hit the "go advanced" button under the reply window, you get a lot more options. One of which is a paper clip looking one that allows you to upload files. I am not sure on size limits, but give this a shot if you have not already.
Maybe you can help me out. I have a 4mb pdf file I'd like to upload. Instructions for adding attachments indicate existence of a "manage attachements" button under the message box. I don't see it. Is it possible that special permissions are required for this function to appear? thanks.