请问用ajax上传文件怎么上传不了呀??
我的代码如下,帮忙看一下

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function post(){
var f = document.fo;
var sn = f.sn.value;
var sf = f.sf.value;

var url = "http://127.0.0.1/postadd.php";

var postStr = "sn="+ sn +"&sf="+ sf ;

var ajax = InitAjax();
ajax.open("POST", url, true);
ajax.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
ajax.send(postStr);
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