How about a "view files" kinda like Google does with google drive & github, there is no need to have a thumbnail just basic viewing can help understand the customer if the game really is DRM-free since all files should be easy to view.
That wouldn't really help, DRM is in the code. And even if the devs were willing to upload the source code (they're probably not), who's gonna spend months reading a game's source code to find this out ;)
