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#10
For the record, I could not get thumbnails of video files, no matter what.
Apparently codecs have to installed in order to show them properly.
I installed K-Lite Basic pack, since I use VLC, I did not have any.
I set "Movies & TV" as default, uninstalled codecs, restarted and thumbnails still show, deleting the cache killed it.
Deleting cache repeatedly, thumbnails are just gone, no matter, what app is set as a default.
Reinstalling codecs without deleting the cache or restarting explorer, thumbnails are back.