What are you trying to offer with the Downloads app?
It is usually used for software, videos and things like that. That’s why “screenshots” makes sense as representation of the content.
If you add images as downloads, then it would be a different story. And the software can’t always create screenshots. For example, I have a website where the downloads consist of images, but they are huge TIF files put into ZIP archives. It would be pretty much impossible for the server to generate thumbnails from that. So, a user field to add some representative images makes perfect sense.