How to disable blendfile thumbnails
First let's unregister Blender's DLL that generates thumbnail. For that, we will use a tool integrated in Windows: Regsvr32, which is designed to unregister DLLs.
Open the command prompt and type this command:
Regsvr32 /u BlendThumb.dll
This should popup if successful:
Then, to get rid of the thumbnails that have already been generated, the simplest way is to reset Windows' entire thumbnail cache. Still in the command prompt, write these lines one after the other:
cd /d %userprofile%\AppData\Local\Microsoft\Windows\Explorer
attrib -h thumbcache_*.db
After that, you can restart Windows and you should not see Blender's files thumbnails anymore while other files' thumbnails will be remade as you go.
If you want to keep the thumbnails off, do not use Blender's installation executable anymore, install new Blender versions only via the compressed zip files.
How to get the thumbnails back
If you install Blender via its executable MSI file, this process includes the registration already.
Alternatively, you can open Blender, go to
Edit menu >
System tab >
Operating System Settings and hit the
Make Default button:
Or, if you like command lines:
And if like me you use Dotbow's Blender launcher, just to RMB > Register Extension:
No need to reset the thumbnail cashe, you might need to just refresh your current opened folders with F5 but the new thumbnails will be generated on the go.