[Linux] Make file filters case insensitive.

[bruvzg]

While file system might be case-sensitive, file extension filters usually an't expected to be.

[fire]

I'm curious how this handles the infamous turkish i.

[bruvzg]

I'm curious how this handles the infamous turkish i.

It's not in the case mapping so won't be converted, but it's probably irrelevant to this use case, I have never seen file extensions that aren't basic Latin.

[akien-mga]

Thanks!