Hi,
Thanks for your reply. I don't want to show the field on the checkout process. I want to show it on the orders details page of the user.
when the user logs in, if he/she clicks the order history page, and then he/she clicks the order number to see its details, i want the user to be able to upload a file (proof of payment).
what i can do now is i can upload from the /administrator side and just show it on the order details page.
if you can look at the screenshot, it only shows "Additional Information" but user can't upload a file.