Add option to enable MIME sniffing

[makew0rld]

I understand that in Caddy v2, MIME sniffing is "disabled by default". But I haven't found a way to enable it. It would be great if Caddy had this feature. In my case for example, I am serving a directory of files without extensions, named after their hash. Sending Content-Type would make it easier for clients to handle the files they receive and display them in the browser if needed.

Thanks!