i want move my attachment to anoter site
 

i have 2 site that i store my file in one and i want store attachment in other owen site(in fact i want enlargement my space of attachment but i dont want move my forum script move.
let me explain my need:
i want when i attach a file to my post it automaticaly move to folder of other site and store with .attach extention and when a user(only user of my site not unregistered user)want,can download it and can access to that file.(in fact i dont want abuse by leechers).
if you have any suggesion please let me know, and if there is a script that provide my purpose,please say or write any link.
thanks a lot