PEAR HTTP_UPLOAD ajax处理时file为空

Question

//ajax上传
var url='upload_js.php';
var pars=new Object();   
    //分别取各表单控件的值，存到对象pars的属性中
    //boy,action
   //得到upfile的值
    pars.asd=$F('upfile');
    pars.action='addfile';
    //实例化ajax对象
  var request = new Ajax.Request
(
    url,
    {
             method:'post',
             evalJS:true,
             parameters:pars,
                 contentType:  'application/x-www-form-urlencoded',
   onComplete:onUpload
    }
 );//upload_js.php
require_once "HTTP/Upload.php";
$upload = new HTTP_Upload();
$destdir = 'upload';
$file = $upload->getFiles('upfile');
//print_r($file)
print_r($file)为空
print_r($_FILES)为空