Nice work. You've cleverly used bitcoin
The only problem is that users don't have bitcoins but they have paypal.

I propose this as a solution:
Let users register.
They'll have a balance of 0.
Let them top up the balance by paying you in paypal or bitcoin.


When they want to download subtract the fee from their balance.

Paying per file could be the next thing..
Competition between uploaders would go to a whole new level

Edit:
I found it to be very difficult to get bitcoins fast outside of the United States.