Usually you want to use the web server where you host your website. That's probably a hosting company, or your Internet provider. If that is the case, you will have to contact them because you can't modify it. I'd
test it first without adding the MIME type though, because chances are that it's already setup correctly.
If you do your own hosting, then you are most likely using Apache, Microsoft's IIS or some other web server (like
Tomcat). Since you are quite unaware of what a web server even is, I doubt this is the case.