HttpRequestMessage Content Disposition null when unit testing

I am trying to write tests for a custom MultipartMemoryStreamProvider - one that is very similar to this MultipartFormDataMemoryStreamProvider.cs

In particular, I am trying to test my own implementation of the GetStream(HttpContent parent, HttpContentHeaders headers) method.

It requires an HttpContent and HttpContentHeaders.

To achieve this I am trying to create a controller context and controller, then pass through the appropriate properties from that controllers request.

In fact, I have tried to implement the answer on this (duplicate) question: Testing a Web API method that uses HttpContext.Current.Request.Files?

Everything I try results in the Content-Disposition on the headers being null As shown in the images below:

Any idea what I am missing?

For code-sake, here is a copy of the code. You will notice it's the same as that in the answer on the other question. I just can't get passed the null content-disposition.

  var content = new ByteArrayContent(new Byte[100]);
        content.Headers.Add("Content-Disposition", "form-data");
        var controllerContext = new HttpControllerContext
        {
            Request = new HttpRequestMessage
            {
                Content = new MultipartContent { content }
            }
        };
        var controller = new MockController();
        controller.ControllerContext = controllerContext;

MockController is simply:

public class MockController : ApiController { }