passing form data as well as file using ajax to controller method

Question

I am trying to pass my form data along with image file from my form to controller method. I got reference from the links: https://qawithexperts.com/article/asp.net/file-uploading-using-jquery-ajax-in-mvc-single-or-multiple-f/98

How to Pass Image File and Form Data From Ajax to MVC Controller

What I did so far is:: My form:

<div class="col-md-4">
    <div class="form-group ">
        <label class="required title_header">Owner Name: </label>
        @Html.EditorFor(model => model.PropName, new { htmlAttributes = new { @class = "form-control" } })
        @Html.ValidationMessageFor(model => model.PropName, "", new { @class = "text-danger" })
    </div>
</div>
// other form data
<div class="col-md-4">
    <div class="form-group ">
        <label class="required title_header">Image</label>
        <input type="file" class="form-control" id="ImageFile" required name="ImageFile" accept="image/*">
    </div>
</div>

<script>
$(document).ready(function () {
        $('#btn_submit').click(function () {
            var testModel = new FormData();         
            var PropName = $('#PropName').val();
            var PropAddress = $('#PropAddress').val();
            var PropContact = $('#PropContact').val();
            var Services = $('#Services').val();            
            var ImageFile = $('#ImageFile').get(0).files;
            var firmModel_ = {
                PropName: PropName,
                PropAddress: PropAddress,
                PropContact: PropContact,
                Services: Services
            }           
            testModel.append("firmModel_", JSON.stringify(firmModel_));
            testModel.append("ImageFile", ImageFile[0]);
            $.ajax({
                type: "POST",
                url: "/FirmModels/Index",
                data: testModel, //JSON.stringify(testModel),
                contentType: false, //"application/json;charset=utf-8",
                processData: false,
                dataType: 'json',
                success: function (response) {
                    if (response.success) {
                        console.log(response.responseText);
                    }
                },
                error: function (response) {
                    alert("failed...");
                }
            })
            return false;
        })
    })
</script>

And my controller code:

[HttpPost]
public JsonResult Index(FirmModel firmModel, HttpPostedFileBase ImageFile, string firmModel_)
{
    // code block
}

While debugging the controller code, the firmMOdel is null, the ImageFile parameter is gettting {System.Web.HttpPostedFileWrapper} and firmModel_ parameter is getting data as:

{"PropName":"raaj vaidaya","PropAddress":"HP","PropContact":"8195551590","Services":"lubricants suppliers"}

So I have some questions regarding the process,

1. why data is not passing to the controller as an object, I mean why FirmModel firmModel is being NULL?
2. The data I am getting in the string firmModel_ , how to use this data to save in database?
3. Is there be any security issues when data is passing to the controller as string format?

Please help, thank you!!!