Hi,
With the classical file custom field, it is not possible.
I think that the best is to edit dynamically the DOM to find the hidden input and change is value.
We have improved the "ajax file" (and "ajax image") custom fields to include this feature. It will be included in the next HikaShop release.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.