$(document).ready(function () {
    $("#btnSubmit").click(function (event) {
        //stop submit the form, we will post it manually.
        event.preventDefault();
        // Get form
        var form = $('#fileUploadForm')[0];
        // Create an FormData object
        var data = new FormData(form);
        // If you want to add an extra field for the FormData
        data.append("CustomField", "This is some extra data, testing");
        $.ajax({
            type: "POST",
            enctype: 'multipart/form-data',
            url: "/api/upload/multi",
            data: data,
            processData: false, //Important, it prevent jQuery form transforming the data into a query string
            contentType: false,
            cache: false,
            success: function (data) { 
                console.log("SUCCESS : ", data);//your logic
            },
            error: function (e) {
                console.log("ERROR : ", e); //your logic
            }
        });
    });
});