File posted to MVC Controller is null

I want to post three things to my MVC Controller: one image and two strings.

On the View, I've got a form that uses enctype="multipart/form-data" that automatically submits the form after an image file is selected. This is the submit handler for this form:

    $("#PhotoUploadForm").on("submit", function (event) {
        event.preventDefault();

        var ImageData = $("#PhotoUploadFileInput").val();
        var GuestNumber = $("#GuestID").val();
        var TCSA_ID = vm.GetSelectedTreatmentAreaTCSA_ID(vm.Photographs.SelectedTreatmentArea());

        var dto = {
            ImageData: ImageData,
            GuestNumber: GuestNumber,
            TCSA_ID: TCSA_ID
        }

        $.ajax({
            url: 'SaveImage',
            type: "POST",
            contentType: "multipart/form-data",
            data: ko.toJSON(dto),
            success: function (data) {
                console.log(submitted);
            }
        });
    });

The dto object is defined in my Model:

public class PhotoUploadDTO
{
    public HttpPostedFileBase ImageData { get; set; }
    public string GuestNumber { get; set; }
    public string TCSA_ID { get; set; }
}

And in my Controller, I have an action that takes in dto as a parameter:

   public ActionResult SaveImage(PhotoUploadDTO dto)
   {
     //etc.
   }

When I try to post dto, everything gets posted as null. This problematic for me because I want to be able to post the image and two strings to the controller simultaneously.

I suspect that the issue is with var ImageData (which is set to the value of <input type="file" id="PhotoUploadFileInput"> on my View), and that it is being posted as C:/fakepath/etc. but not as the actual image file. It's frustrating because I know it wouldn't even be an issue if I had a form that just posted the image, but I need to use this submit handler and I don't know how to bring the actual image data into it.

Why is the data null when it hits the MVC Controller, and how can I post these three items while still being able to use a submit handler?