If there are more elements of the same product in the carts, the plugin default behavior provides the file upload only once: therefore the files will be the same for all the copies of the product.
If the option “Splt Products” is enabled, you will find in cart one line for each item of the same product. This allows users to upload different files for each item added to the cart.
This splitting happens only if users edit product quantity from product page, while if you change quantity in cart, items will not be splitted as explained above, but standard WooCommerce behaviour will apply. If you change quantity from cart, you will be able to purchase more copies of the same product (attachments included).