从零开始创建FormData对象

var formData = new FormData();
 
formData.append("username", "Groucho");
formData.append("accountnum", 123456); //数字123456会被立即转换成字符串 "123456"
 
// HTML 文件类型input，由用户选择
formData.append("userfile", fileInputElement.files[0]);
 
// JavaScript file-like 对象
var content = '<a id="a"><b id="b">hey!</b></a>'; // 新文件的正文...
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);