new FormData() 相关用法

 1.simple demo :

var form = document.forms.namedItem("fileinfo");
form.addEventListener('submit', function(ev) {

  var oOutput = document.querySelector("div"),
      oData = new FormData(form);//form just origin js object

  oData.append("CustomField", "This is some extra data");

  var oReq = new XMLHttpRequest();
  oReq.open("POST", "stash.php", true);
  oReq.onload = function(oEvent) {
    if (oReq.status == 200) {
      oOutput.innerHTML = "Uploaded!";
    } else {
      oOutput.innerHTML = "Error " + oReq.status + " occurred when trying to upload your file.<br \/>";
    }
  };

  oReq.send(oData);
  ev.preventDefault();
}, false);

2.referer:更复杂的其它方式 上传文件和表单

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Using_XMLHttpRequest#Submitting_forms_and_uploading_files