form_upload.js

var formElement = document.getElementById("cart-form");
var formAction = "/cart";
var formMethod = "POST";

var formData = new FormData(formElement);
var file = load_your_file_here;

formData.append('properties[file]', file);

var xhr = new XMLHttpRequest();
xhr.open(formMethod, formAction);
//xhr.setRequestHeader("X-Requested-With", "XMLHttpRequest");
xhr.send(formData);