If you are dragging over the email my guess is its creating a link to a document and respecting the security of your mail file. Nothing you can do about that.
I don't think you can put attachments into the RTfield but you can create an upload/download control to put the files on the document.
You might also be able to setup the NSF as a mail in db. The user can forward the email into the database, it would then be able to be opened and modified.