I have configured a downloadable product by using product attributes

So in brief I have just one attribute which is always the same. I have configured my store to show Add to cart -buttons next to the products in the product listing page.

However, with my downloadable products, add to cart -buttons are not showing. There is just text more information. Can I somehow make the only attribute the default so that zen cart would allow showing the add to cart button also next to my downloadable products?

My Zen cart version is 1.3.8.

Thanks in advance!