How does Response.ContentType work?

Question

I am downloading an xml string from ASP.Net and want it to open in MS Office Infopath. I set the ContentType to "application/ms-infopath.xml" (not sure where I got that from).

On some machines it is fine, on others the xml opens in the browser, or in Visual Studio or some other xml editor, even though Infopath is present.

1. Who keeps the list of valid content types?
2. What does the OS or browser do with it?
3. How can I force this XML to open in the program of my choice (Infopath)?