Suppose your Ajax form like this:

@using (Ajax.BeginForm("UploadFile", null, new AjaxOptions { HttpMethod="POST", UpdateTargetId = "result" }, new { enctype = "multipart/form-data" }))
{         
  <label id="lblUploadNewFile" for="fileUploadControl">Upload New File</label>     
  <input type="file" name="fileToUpload" id="fileUploadControl"/>
  <input id="btnFileUpload" type="submit" value="Upload" />
  <span id="result" />            
}

If you want to post a file using ASP.NET MVC Ajax form?, you need to add the following line of coding in the same page of your Ajax form.

window.addEventListener("submit", function (e) {
    var form = e.target;
    if (form.getAttribute("enctype") === "multipart/form-data") {
        if (form.dataset.ajax) {
            e.preventDefault();
            e.stopImmediatePropagation();
            var xhr = new XMLHttpRequest();
            xhr.open(form.method, form.action);
            xhr.onreadystatechange = function () {
                if (xhr.readyState == 4 && xhr.status == 200) {
                    if (form.dataset.ajaxUpdate) {
                        var updateTarget = document.querySelector(form.dataset.ajaxUpdate);
                        if (updateTarget) {
                            updateTarget.innerHTML = xhr.responseText;
                        } 
                    }
                }
            };
            xhr.send(new FormData(form));
        }
    }
}, true);