如何在form中动态添加输入框

<body bgcolor="#FFFFFF" text="#000000">
<form name="form1" method="post" action="upfile.asp" enctype="multipart/form-data" >
  <table border="1" cellspacing="0" cellpadding="0" bordercolorlight="#000000" bordercolordark="#CCCCCC" width="91" height="23">
    <tr> 
      <td align="left" valign="middle" height="18" width="18">&nbsp;</td>
      <td bgcolor="#CCCCCC" align="left" valign="middle" height="18" width="67"> 文件上传</td>
    </tr>
  </table>
  <br>
  <input type="hidden" name="act" value="upload">
  <br>
  <table width="71%" border="1" cellspacing="0" cellpadding="5" align="center" bordercolordark="#CCCCCC" bordercolorlight="#000000">
    <tr bgcolor="#CCCCCC"> 
      <td height="22" align="left" valign="middle" bgcolor="#CCCCCC">&nbsp;化境编程界文件上传</td>
    </tr>
    <tr align="left" valign="middle" bgcolor="#eeeeee"> 
      <td bgcolor="#eeeeee" height="92"> 
        <script language="javascript">
  function setid()
  {
  str='<br>';
  if(!window.form1.upcount.value)
   window.form1.upcount.value=1;
    for(i=1;i<=window.form1.upcount.value;i++)
     str+='文件'+i+':<input type="file" name="file'+i+'" style="width:400" class="tx1"><br><br>';
  window.upid.innerHTML=str+'<br>';
  }
  </script>
        <li> 需要上传的个数 
          <input type="text" name="upcount" class="tx" value="1">
          <input type="button" name="Button" class="bt" onclick="setid();" value="· 设定 ·">
        </li>
        <br>
        <br>
        <li>上传到: 
          <input type="text" name="filepath" class="tx" style="width:350" value="">
        </li>
      </td>
    </tr>
    <tr align="center" valign="middle"> 
      <td align="left" id="upid" height="122"> 文件1: 
        <input type="file" name="file1" style="width:400" class="tx1" value="">
      </td>
    </tr>
    <tr align="center" valign="middle" bgcolor="#eeeeee"> 
      <td bgcolor="#eeeeee" height="24"> 
        <input type="submit" name="Submit" value="· 提交 ·" class="bt">
        <input type="reset" name="Submit2" value="· 重执 ·" class="bt">
      </td>
    </tr>
  </table>
</form>
</body><script language="javascript">setid();
</script>