iframe.html

<iframe id="upload_target" style="display:none;width:0;height:0"></iframe>

<form action="<upload handler>" target="upload_target" enctype="multipart/form-data" method="post">
	<input type="file" name="file" />
</form>

old.html

<form action="<upload handler>" enctype="multipart/form-data" method="post">
	<input type="file" name="file" />
</form>

xhr2.js

function sendForm(form) {
	var formData = new  FormData(form);
	var req = new XMLHttpRequest();
	req.open("POST", "<upload handler>");
	req.send(formData);
	return req;
}