It's been a while since I used Krita and I'm fresh after formatting Windows 10, everything's mostly fine, except when opening .kra files the thumbnail is busted. It just shows Krita's icon. I have latest Krita, with Shell Extension (both reinstalled), and the "Enable native file dialogue" enabled as well. I also unchecked "show icons instead of thumbnails" in W10 folder menu and also cleared the cache. It's really driving me nuts and makes any work extremely frustrating >:V

I noticed it "kind of" works. When I modify a kra file in Krita, save it, and open the file window when it's still saving, it will actually display the thumbnail properly. Though that only lasts for a few seconds.
https://cdn.discordapp.com/attachments/743444596029259786/743917979120566352/Help_Pic.png

That's probably the moment the mergedimage.png file is saved in the zip file; you don't say how you've installed Krita, through the Windows Store, the installer or the portable zip file, but you can try to just install the thumbnailer: https://download.kde.org/stable/krita/k ... -setup.exe

I downloaded Krita through the Windows Installer, from the official page. Unfortunately, the file you provided just says I use a newer version of the Shell Integration (1.2.4.2 versus the 1.2.3.0 in the link).

In that case, I have no idea. There's something wrong with your windows installation or settings, and I cannot solve that for you, sorry.