I really don't know why that's not working for you. I assume that RadarBox sends the file as RadarBoxScreenShot.gif, but that the Gofreeserve server names it as a temp file until it receives all of the file, when it renames it to the proper name.
My Gofreeserve folder collects TMP files at apparently random times (possibly if I close RadarBox during FTP upload) and I go in and delete them every few weeks.
Perhaps we need an FTP expert (which I'm not) to solve your problem.
Rod