This took me a while to figure out after my most recent Windows 10 update, so putting on my blog partly so I can always find it and just maybe it’ll help some other people.
Basically, when only some portion of thumbnails breaks, it’s an indication that Explorer has forgotten how to render thumbnails for those file types. We just have to re-teach it.
In my case, I lost both PNG and BMP previews, but JPG were fine. Go figure. So, fire up your trusty regedit. We’re looking for a path like:
Computer\HKEY_CLASSES_ROOT\.png\shellex\{E357FCCD-A995-4576-B01F-234630154E96}
Under HKEY_CLASSES_ROOT, find your troublesome extension. In that weird GUID folder, you’ll find only one key marked (Default). Somehow, in an update, it got set to {9EE0C242-8973-456D-B382-0752476703FD}.
Change that to {C7657C4A-9F68-40fa-A4DF-96BC08EB3551}
That’s it. You can reboot, and everything will be fine. (Technically, you can kill and restart explorer if you’re savvy to that, but a restart is easier.)
(More info: https://docs.microsoft.com/en-us/windows/desktop/wic/-wic-integrationregentries)