You can click button "Downloadable Files", and then upload file. If you want to use the uploaded files later, you only need click it and then insert it to editor area, othere users will can right click the file and save to their local computer.
If you want to click one button and then select some files and then send it, you can use the combination of ajax uploader and cuteeditor, please refer to
http://cutesoft.net/example/Online-email.aspx, this is cuteeditor, you can add one ajax uploader instance to this page
Regards,
Eric