[Suggestion] Add option to invert "Ignored File Extensions" into a whitelist

[enzomtpYT]

Hello, while using the app i noticed there was an option for Excluding some File Extensions, but i didn't find anything to "invert" that list into a white list that only Allows some File Extensions. That's why im suggesting it as a kind of tickbox.

[CyanVoxel]

Implemented in #251