Hi,
You can temporally change the custom field type and switch it to "text" : so you will be able to edit the product and remove the data.
You can also use the browser inspector to change the data stored in the input hidden field.
Otherwise, we generally recommend to use the "ajax file" field which provide more features.
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.