We do not support this type of modification of power store.
you would need to edit the webassist/plugins/shopping_cart/addtocatrt.php page to add the file field.
use the file manipulation -> Upload Behavior of Universal Email.
edit the eCart object of power store using eCart to add a new cart column. edit the add to cart behavior webassist/plugins/shopping_cart/addtocatrt.php page to store thew uploaded file name to the cart.