In testing I replaced the file shopping_cart.php with one from 1.5.7C (unmodified) and I tested adding to cart a product with no options selected. I was not able to add and it asked me to make sure I select an option. I tested with a product that requires an uploaded file and I selected A JPG which is not permitted and the product was added to the cart without the attribute.

I can only conclude that indeed a code check is missed for file upload.