To server a host-meta file in the .well-known folder from an ASP MVC application hosted in IIS, the easiest way is probably saving your host-meta file in a folder then adding a route and action for it.
So this would be your route in Global.asax
routes.MapRoute("HostMeta", ".well-known/host-meta", new {controller = "WellKnown", action = "HostMeta"});
and here is the new controller to serve it:
public class WellKnownController : Controller
{
public ActionResult HostMeta()
{
string result = System.IO.File.ReadAllText(Request.MapPath("~/somewhere-on-the-server/host-meta"));
return Content(result);
}
}