介绍三种上传方式：
真正的异步上传，FormData的更多操作，请点击 这里
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<title></title>
</head>
<body>
<input type="file" id="FileData" />
<br />
<input type="button" id="btn1" value="上传" />
<script>
btn1.onclick = function () {
var formData = new FormData();
formData.append("FileData", FileData.files[0]);
var request = new XMLHttpRequest();
request.open("POST", "Handler1.ashx");
request.onload = function (e) {
if (request.status == 200) {
var res = e.target.responseText;
}
};
request.send(formData);
};
</script>
</body>
</html>
后端代码，跟 文件上传一：伪刷新上传 一样。