Set specific upload rules to a product
Generally setting some global upload rules could be enough for general purpose shops, but you could need to customize the upload rules for your products one by one by setting the number of files allowed or the file type or the file size.
You can set the ‘upload rules’ for a specific product from the edit product page, look at the following picture:

Try adding this product to the cart in order to see that, only for this product, your customer can upload two files, labeled as ‘Front picture’ and ‘Back picture’.