by handing over variables... lets say the IP and or timestamp..
now make the file you have the header in check if the info is correct.. tada..
check out
http://joskeonline.com/~keepmyfi/
the real site (
http://www.keepmyfile.com) only accepts small files for now since it just moved to a new server.. still have to set upload size in php.ini .. thats on my to-do-list for tomorrow
try hotlinking an uploaded image of yours (the real ones.. thumbs are intended for hotlinking)