Buyer submission of Images, files or feedback/comments in MarketPress checkout

I’d like to see buyers have the ability to upload image(s) during the checkout process.

In particular, I’ve a client who does screen printing and would like to allow buyers to upload images (preferably within a list of approved filetypes) to be put on tees and other merchandise. I’m sure there are other applications for this like, maybe, a copy/printing shop or cd duplication…actually there are a lot of uses.