files: add setting to whether append header X-Content-Type-Options: nosniff

[KisaragiEffective]

Hello! I think it would be useful when publishing files on local, which have non-major mime. If X-Content-Type-Options: nosniff was not set, modern browser will not load them in certain cases (I've hit this when call WASM initialization from JavaScript).

So, it's amazing if we can choice whether insert the header or not.

Thank you ;)