In thumbs mode, images thumbs are shown. Videos are not (sadly) and folders only show a very austere icon:
Google Photos, instead, shows a merged preview of a couple of selected contained pictures.
While selection is hard/arbitrary (random, first, last, face-detection, …) having at least of couple of random one would be really nice.
Imagemagick example: https://stackoverflow.com/questions/37709879/ (They call that a “montage”).