How do I unit test a custom ActionFilter in ASP.Net MVC

You just need to test the filter itself. Just create an instance and call the OnActionExecuted() method with test data then check the result. It helps to pull the code apart as much as possible. Most of the heavy lifting is done inside the CsvResult class which can be tested individually. You don’t need to test the filter on an actual controller. Making that work is the MVC framework’s responsibility.

public void AcceptsTypeFilterJson_RequestHeaderAcceptsJson_ReturnsJson()
{
    var context = new ActionExecutedContext();
    context.HttpContext = // mock an http context and set the accept-type. I don't know how to do this, but there are many questions about it.
    context.Result = new ViewResult(...); // What your controller would return
    var filter = new AcceptTypesAttribute(HttpContentTypes.Json);

    filter.OnActionExecuted(context);

    Assert.True(context.Result is JsonResult);
}