Doesn't seem to be a problem with Internet Explorer/Windows Explorer. I just uploaded a couple of test text files with it into my RB433. In the address bar of IE:
It should ask for a user/password. To upload anything, I had to open the ftp site in Windows Explorer. To do that in my version, it is under "Page" on the menu bar.
Then it is "copy and paste" time! Highlight the filename, right-click on the highlight, and click "copy". Then paste into the MT box window.
BTW, the user/password is sent unencrypted. Just so ya know.