Note: T320510 will simply remove non-NS_FILE thumbnails again from Commons (and other non-Wikipedias), but they will still be displayed to fill the square (with edges cut)

Just a bit of context about why Special:Search thumbnail presentation has changed (in general): the traditional way of displaying them is visually inconsistent, and distracting.
We felt that adding relevant thumbnails to article results (not talking about Commons here) could greatly improve the search experience since they add additional context for people trying to find a matching result, but they shouldn't distract from the main content of the search results.

Of course, on Commons, that main content *is* the files, and being able to see them in full is of course not a distraction (quite the opposite - not being able to see them in full is painful)
That said, all our projects share a codebase, and these changes also ended up affecting Commons.

It looks like User:Speravir already came up with the same CSS fixes I was about to propose in order to restore thumbnail presentation on Commons (note: I would also recommend removing the bounding box border - that wasn't there before either)
I would highly encourage a Commons admin to add this to Common.css to apply this site-wide!

.searchResultImage-thumbnail .image img {
    object-fit: contain;
    object-position: center;
    border: none;
}

Thanks, I have made a request for somebody to add that piece of code to Commons' Common.css.

In T320459#8316401, @El_Grafo wrote:

Thanks, I have made a request for somebody to add that piece of code to Commons' Common.css.

Added now: https://commons.wikimedia.org/w/index.php?title=MediaWiki:Common.css&diff=696305350&oldid=563766287 and works well.

Thanks @matthiasmullie for the snippet.

Great, thanks everyone!