Routing optional parameters in ASP.NET MVC 5

Maybe you should try to have your enums as integers instead?

This is how I did it

public enum ECacheType
{
    cache=1, none=2
}

public enum EFileType 
{
    t1=1, t2=2
}

public class TestController
{
    [Route("{type}/{library}/{version}/{file?}/{renew?}")]
    public ActionResult Index2(EFileType type,
                              string library,
                              string version,
                              string file = null,
                              ECacheType renew = ECacheType.cache)
    {
        return View("Index");
    }
}

And my routing file

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

    // To enable route attribute in controllers
    routes.MapMvcAttributeRoutes();

    routes.MapRoute(
            name: "Default",
            url: "{controller}/{action}/{id}",
            defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional });
}

I can then make calls like

http://localhost:52392/2/lib1/ver1/file1/1
http://localhost:52392/2/lib1/ver1/file1
http://localhost:52392/2/lib1/ver1

or

http://localhost:52392/2/lib1/ver1/file1/1/
http://localhost:52392/2/lib1/ver1/file1/
http://localhost:52392/2/lib1/ver1/

and it works fine...