如何动态添加上传控件

<input type="button" name="Submit2" value="增加" onclick="insertRow()">
   <br/>
   <table id="table1" width="500" border="1" cellspacing="0" cellpadding="0">
     <tr>
       <td ><input name=file1 type=file></td>
       <td ><input type="button" name="1" value="减少" onclick="this.parentNode.parentNode.removeNode(true)"></td>
     </tr>

   </table>
<script language=javascript>
function insertRow()
{
var obj=document.getElementById("table1")
var rowIndex=obj.childNodes[0].childNodes.length;
var objTR=obj.insertRow(rowIndex);
var objTD1=objTR.insertCell();
var objTD2=objTR.insertCell();
rowIndex2=parseInt(rowIndex)+parseInt(1)
objTD1.innerHTML=obj.childNodes[0].childNodes[rowIndex-1].childNodes[0].innerHTML
objTD2.innerHTML=obj.childNodes[0].childNodes[rowIndex-1].childNodes[1].innerHTML
}
</script>