为什么ajax回发不能使fileupload文本框为空

<input type=\"file\" id=\"uploadFile\" name=\"uploadFile\" />
<input type=\"button\" id=\"uploadButton\" value=\"Addfile\" onclick=\"uploadfile()\" />

> function uploadfile() {
>       var uploadFile = document.getElementById(\"uploadFile\").value;
>         $.ajax({
>             type: \"POST\",
>             url: \"/controller/actionmethod\",
>             data : \"uploadFile=\" +uploadFile
>             
>           });

    }