How to Post file uploader object from knockout viewModel to mvc HttpPostFileBase object

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MVC4

Knockout.js

i am working with knockout.js. i post a form using knockout to mvc controller in the form i use a file upload. when i post ViewModel every thing is post properly but the file object always null. any one help me how to pass file object in httpPostFileBase.



This is UI

<script type="text/javascript" src="~/Scripts/App/AdminScript.js"></script>
<script type="text/javascript" src="~/Scripts/App/fileBindler.js"></script>
<script type="text/javascript">
    $(document).ready(function () {
        window.ko.applyBindings(new CategoryViewModel());

    });
</script>
    <div id="divEditCategory">
        <h2>Edit Category</h2>
        <div data-bind="with:currentCategory">
            <p>Category Name: <input type="text" placeholder="Category Name" class="form-control" data-bind="value:CategoryName" /></p>
            <p>Category Name: <input type="file" id="imgPath" accept="image/*" data-bind="value: ImagePath" /></p>
            <p><input type="checkbox" data-bind="value:IsActive" />Is Active</p>
        </div>
        <p><input type="button" id="sendCategory" class="btn btn-success" value="Save" data-bind="click: saveCategory" /></p>
    </div>

And this is my ViewModel

function Category(categoryName, imagePath, isActive)
{
    var fd = new FormData();
    var self = this;
    self.CategoryName = ko.observable(categoryName);
    self.IsActive = ko.observable(isActive),
    self.ImagePath,  = ko.observable(imagePath),
}
function CategoryViewModel()
{
    var self = this;

    self.currentCategory = ko.observable(new Category("","", true));

    self.saveCategory = function () {
        debugger;
        var json1 = ko.toJSON(self.currentCategory);
        debugger;
        $.ajax({
            url: '/Admin/EditCategory',
            type: 'POST',
            dataType: 'json',
            data: json1,
            contentType: 'application/json; charset=utf-8',
            success: function (data) {
                var message = data.Message;
            }
        });
    }

}