-- HikaShop version -- : 2.3.4
-- Joomla version -- : 3.3.6
-- PHP version -- : 5.5.18
-- Browser(s) name and version -- : any
Here's how to reproduce.
1) Attach some file to a product.
2) After that, edit its file-name through the edit file form.
3) Try do download it through the frontend => this results in "file not found" error
4) Check file in Hikashop's upload/safe directory => Still old file name...
5) rename the file directly => Download works again.
Conclusion: the form does not really rename the file, but only the database entry.