ส่ง $_FILES แบบ Ajax อย่างไรครับ คือตัวแปรรับค่า $_FILES(file ) ไม่ได้เลยครับ

function handleResponse_pic () {
	if (objRequest.readyState == 4 && objRequest.status ==200) {
		var data	=	objRequest.responseText;
		document.getElementById("myblock11").innerHTML = data;
		document.getElementById('loading_cus').style.display = "none";
		document.getElementById('id_picture').value = data;
	}
}

function uppicture() {
	document.getElementById('loading_cus').style.display = "";
	var id_order = document.getElementById("id_order");
	var id_product = document.getElementById("id_product");
	var file = document.getElementById("file");
	var id_picture = document.getElementById("id_picture");

	var postBody = 'id_order=' + id_order.value
	+ '&id_product=' + id_product.value 
	+ '&file=' + file.value
	+ '&id_picture =' + id_picture.value;

	objRequest.open('POST', 'save_pic.php?'+ Math.random(), true);
	objRequest.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
	objRequest.onreadystatechange = handleResponse_pic;
	objRequest.send(postBody);
}