[File Explorer] Add a warning that previously generated thumbnails won't disappear when disabled

[jaimecbernardo]

Description of the new feature / enhancement

When one of the thumbnail plugins is disabled, the generated thumbnails won't get deleted, since Windows keeps a cache of these.
This is surprising for users, so an informational text near the enabling/disabling zone for Thumbnails in settings might be useful.

Scenario when this would be used?

Extra Information when enabling/disabling thumbnail generators.

Supporting information

#13549 (comment)

[davidegiacometti]

Related to #7213
As discussed in the issue there is no official API to clear thumbnails for an extension: the warning could be the right choice.
We still have the reboot warning with #12996 merged. It's not clear if under some circumstances a reboot is still required even if SHChangeNotify is called.

[Jay-o-Way]

Sounds like something that needs to be mentioned in the Docs too. Current info (on the explorer page) is minimal by itself and PDF hasn't even been included yet.
See #10878 too.

[takedownca]

I came here to report this "bug". The PDF preview was enabled by default, but no amount of disabling and rebooting makes the icons go back to normal. How do I delete the thumbnails?

[Jay-o-Way]

How do I delete the thumbnails?

Quickest answer would be to clear the icon cache. You could use disk cleanup or CCleaner or something similar.

[franky920920]

How do I delete the thumbnails?

Navigate to %localappdata%\Microsoft\Windows\Explorer and delete files start with iconcache_ and thumbcache_ should clear them

[takedownca]

How do I delete the thumbnails?

Navigate to %localappdata%\Microsoft\Windows\Explorer and delete files start with iconcache_ and thumbcache_ should clear them

Thanks. That did the trick.