Why would IIS 5.1 (Windows XP) not produce the correct MIME type for css files?

Question

I'm doing some HTML pages which include a style sheet and some server side includes. Now, I want to set up a laptop so I can work on it; the machine has Windows XP and IIS 5.1. I have set up a virtual directory; the SSIs load. On the virtual directory I've set up the mime type for .css to be text/css (see the screenshot of the dialog below).

IIS still serves the header as text/html. Any ideas why?

HTTP/1.1 200 OK
Server: Microsoft-IIS/5.1
Date: Mon, 05 Dec 2011 22:31:24 GMT
X-Powered-By: ASP.NET
Content-Type: text/html