As far as I know from searching in the forum, up to know it is not possible to let customers upload files to the nopCommerce store.
Maybe this would be a nice feature to add in a further release version of nopCommerce and a way to differ from other online shop solutions out there. There might be some interesting ideas for nopCommerce users to let costumers upload files, such as documents/pictures/soundfiles or even video.
I do not know how hard it is to implement this feature but it would be much more than just a "nice to have" thing.
Just some input - greetinx and big thanks for your good work on nopCommerce! :)