ASP.NET-MVC How to serve static content?

Basically I want to have a static website, but I did this and an ASP.NET-MVC application to serve a simple HTTP mail request to save something to the database. Sometimes in the future a view may be implemented, but not at this time. What I found is some strange behavior that I cannot explain why?

Site structure:

Root/
    index.html
    test.html
    img/
        image.png
        test.html

      

        
        
        
      

    

The server is hosted on an IIS 8.9 server.

Route configuration and controller:

public static void RegisterRoutes(RouteCollection routes)
{
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapRoute(
             name: "ApiPostData",
             url: "api/{action}",
             defaults: new { controller = "Api" }
        );
}

public class ApiController : Controller
{

    [HttpPost]
    public JsonResult PostSomeData(SomeData someData)
    {
            // save to database
            return Json(true);
    }
}

      

        
        
        
      

    

But it behaves completely wrong and unpredictable: In the examples:

www.mysite.com -> displays index.html OK

www.mysite.com/index.html -> error 500

www.mysite.com/img/image.png -> shwon image which is ok

www.mysite.com/img/test.html -> error 500

www.mysite.com/api/postsomedata -> calls the controller method as it should

This is completely illogical. Why does the server make a distinction between static context. I need an explanation of this behavior and a way to get IIS to serve static content.