Change the filename of submission in assignment

Hi,

I have one problem. I use PDF submission. After the student send file to grade, I have plugin for import this file to student's folder, where could be all files from this student. But problem is, when teacher grade this file and send it back to edit and then student edit it and send by same filename, my import plugin rewrite the old file with this new one. But I need in student's folder all of files to check any changes in that files. Any idea how I can change the filename of submisson, for example adding date time to filename (it could be 'submission 20160405.pdf')? It would be very helpfully for my work.

Cheers,

Dominik Zatloukal