by Alfredo Ruiz.
I think it works in a different way. It doesn't "overwrite" the uploaded files. You just upload a file, or a few files it doesn't really matters. If you need to send a new one, just delete the old one and upload it again. The point is that you don't see the "delete" option very easily. You have to right-click on the file icon, a context windows appears, and one of the options is "delete". You may also change the properties of the file. So the students need to know how to do it, of course. I hope this works for you.