What settings do you have under configuration->attributes_settings?

Does the problem exist with all downloads, or just this one?

What permissions do you have for your /pub/ folder?

What permission do you have on your /downloads/ folder?

Although probably not a *cause* of the problem, your screenshot is indicating that the size of this file is only 152bytes. The just happens to be the size of a zip file that is created when zipping a file with a single character filename with no content (IOW, an empty or unreadable file). Coincidence? Probably not.

Hope this helps. If not, you'll need to answer those questions so that we have the same clues that you do :)

Cheers
Rod