FormData

var formData = new FormData();

formData.append("username", "Groucho");
formData.append("accountnum", 123456); // 123456  "123456"

// HTML  input
formData.append("userfile", fileInputElement.files[0]);

// JavaScript file-like 
var content = '<a id="a"><b id="b">hey!</b></a>'; // 
var blob = new Blob([content], { type: "text/xml" });

formData.append("webmasterfile", blob);

var request = new XMLHttpRequest();
request.open("POST", "http://foo.com/submitform.php");
request.send(formData);

var formData = new FormData(someFormElement);

var formElement = document.querySelector("form");
var request = new XMLHttpRequest();
request.open("POST", "submitform.php");
request.send(new FormData(formElement));

var formElement = document.querySelector("form");
var formData = new FormData(formElement);
var request = new XMLHttpRequest();
request.open("POST", "submitform.php");
formData.append("serialnumber", serialNumber++);
request.send(formData);

<form enctype="multipart/form-data" method="post" name="fileinfo">
  <label>Your email address:</label>
  <input
    type="email"
    autocomplete="on"
    autofocus
    name="userid"
    placeholder="email"
    required
    size="32"
    maxlength="64" /><br />
  <label>Custom file label:</label>
  <input type="text" name="filelabel" size="12" maxlength="32" /><br />
  <label>File to stash:</label>
  <input type="file" name="file" required />
  <input type="submit" value="Stash the file!" />
</form>
<div></div>

var form = document.forms.namedItem("fileinfo");
form.addEventListener(
  "submit",
  function (ev) {
    var oOutput = document.querySelector("div"),
      oData = new FormData(form);

    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,
);

data.append("myfile", myBlob, "filename.txt");

var fd = new FormData(document.querySelector("form"));
fd.append("CustomField", "This is some extra data");
$.ajax({
  url: "stash.php",
  type: "POST",
  data: fd,
  processData: false, // 
  contentType: false, // 
});