Why do none of my ActionFilters run?

I asked a question earlier today about ActionFilters in ASP.Net MVC. It turned out my problem was really that my ActionFilter is not even running. Among other things I read this article, and I can't find anything he does that I don't.

This is my code:

// The ActionFilter itself
public class TestingIfItWorksAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        filterContext.Controller.TempData["filter"] = "it worked!";
        base.OnActionExecuting(filterContext);
    }
}

// The Controller Action with the filter applied
[TestingIfItWorks]
public ActionResult Test()
{
    var didit = TempData["filter"];
    return View();
}

A breakpoint in the filter method is never hit when I debug, and TempData["filter"] holds a null value when the view is rendered.

Why is this not working?