Hi,
When you have an "item" custom field ; you are able to see the attached file in your HikaShop backend.
The custom field will display a link and you can click on that link to see/download the file.
Customer can be able to see their item custom field when displaying their order in the front-end.
They can't modify an item custom field after an order has been made ; the fields are only in edition during the checkout.
But using some custom development, it could be possible to make some field editable in the order page. It requires some PHP/Development skills.
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.