POST via ajax para upload file

Eu estou querendo mandar por post file, mas n estou conseguindo. nunca usei ajax via post gostaria de saber qual as configurações necessarias.

// UPLOAD DE IMAGENS

var AjaxRMS;
function AjaxRequest(){
 AjaxRMS = false;
 if (window.XMLHttpRequest) {
 AjaxRMS = new XMLHttpRequest();// Mozilla, Safari
 }
 else if (window.ActiveXObject) {
 try {
 AjaxRMS = new ActiveXObject("Msxml2.XMLHTTP");// IE
 } catch (e) {
 try {
 AjaxRMS = new ActiveXObject("Microsoft.XMLHTTP");// IE
 } catch (e) {}
 }
 }
}

function upload(){

var MAX_FILE_SIZE = document.formUpload.MAX_FILE_SIZE.value;
var userfile = document.formUpload.userfile.value;

AjaxRequest();
if(!AjaxRMS){
return;
}

 var url ="upload.php";//página onde será feita a consulta
 var params = "MAX_FILE_SIZE="+MAX_FILE_SIZE+"&userfile="+userfile;

 AjaxRMS.onreadystatechange = Mostra_dados;
 AjaxRMS.open("POST", url, true);
 AjaxRMS.setRequestHeader("Content-Type", "multipart/form-data");
 AjaxRMS.setRequestHeader("Content-length", params.length);
 AjaxRMS.setRequestHeader("Connection", "close");
 AjaxRMS.send(params);
 }

function Mostra_dados(){
if(xmlhttp.readyState==4)
 {
 document.formUpload.MAX_FILE_SIZE.value = '';
 document.formUpload.userfile.value = '';

 document.getElementById("resposta").innerHTML=xmlhttp.responseText;
 $('#resposta').fadeIn('fast');
 $('#uploaded').fadeIn(1000).hide('slow');
 }
}

// FIM UPLOAD

obrigado