In case that the aseprite-thumbnailer (now included with Aseprite installer) shows only thumbnails for .aseprite files, and not for .ase files. You can:
- Open regedit
- Go to Computer > HKEY_CLASSES_ROOT > .ase
- Check that “(Default)” value is equal to “AsepriteFile”
- If it isn’t, you can double-click it to modify it and set it to AsepriteFile