js控制文件大小

<script type="text/javascript">
    function chksize(strFileName){
        try
        {
         var maxsize = 100;     //定义允许文件的大小，单位KB，请根据需要自行修改！
         var objStream = new ActiveXObject("ADODB.Stream");
         objStream.Type = 1;
         objStream.Open();
         objStream.LoadFromFile(strFileName);
         if(Math.round(objStream.Size/1024)>maxsize)
             {alert ("抱歉！您选择的文件为 "+Math.round(objStream.Size/1024,2)+" KB　n超过了程序"+maxsize+" KB 的限制！");
             document.form1.reset();
             return false;
         }
         else
            alert("可以上传");
        }catch(e)
        {
            alert("不支持");
        }
    }
</script>
<form name="form1" action="" method="post">
<input type="file" name="Upfile" onchange="chksize(this.value)" />