I have set upp a rule in my template-selector class, like this.

            viewTemplateModelRegistrator.Add(typeof(VimeoBlock), new TemplateModel
            {
                Name = "MyTemplate",
                Default = true,
                TemplateTypeCategory = TemplateTypeCategories.MvcPartialController,
                TemplateType = typeof(MyTemplatePartialController),
                AvailableWithoutTag = true,
                Tags = null,
                Path = "~/Views/Shared/Partials/_MyView.cshtml"
            });

The view exits and to make it simple my controller looks like this ->

    public class MyTemplatePartialController : ActionControllerBase
    {
        // GET: MyTemplatePartial
        public ActionResult Index(VimeoBlock currentContent)
        {
            return Content("My content");
        }
    }

Now when I drag a "VimeoBlock" into a xhtml property it renders with the content "my content" as expected. (in html )

BUT if I swap all references from VimeoBlock to ImageMedia (inheritd ImageDate) like this.

            viewTemplateModelRegistrator.Add(typeof(ImageMedia), new TemplateModel
            {
                Name = "MyTemplate",
                Default = true,
                TemplateTypeCategory = TemplateTypeCategories.MvcPartialController,
                TemplateType = typeof(MyTemplatePartialController),
                AvailableWithoutTag = true,
                Tags = null,
                Path = "~/Views/Shared/Partials/_MyView.cshtml"
            });

    public class MyTemplatePartialController : ActionControllerBase
    {
        // GET: MyTemplatePartial
        public ActionResult Index(ImageMedia currentContent)
        {
            return Content("My content");
        }
    }

The result in html is this -> 

Thanks for the help.

PS. I have version 10.4.1 and when comparing to an older project with version 8.x.x I can't see what I do differently and it works in the older project.