在客户端判断上传文件大小(不支持opera)

参考网上写的客户端判断上传文件大小,opera无法支持

    <!doctype html>
<html>
  <head>
  <meta charset = "gb2312" />
    <script type="text/javascript">
        function checkFileChange(obj) {
            var filesize = 0;
            if(navigator.userAgent.indexOf("MSIE")>0){  //if ie
                var filePath = obj.value;
                var image = document.getElementById("img");
                image.dynsrc=filePath;   
                filesize=image.fileSize;    
            }
			else  // if chrome safari firefox
			{
                filesize = obj.files[0].fileSize;
			}
            alert(filesize);
        }
    </script>

  </head>
  <body>

    <input type="file" id="myId" name="myId">
    <input type="button" value="得到文件大小" onclick="checkFileChange(document.getElementById('myId'))">
	<img id = "img" style = "display:none" />
  </body>
</html>