js控制下传文件的大小

<html> 
<head> 
<script type="text/javascript"> 
	var isIE = /msie/i.test(navigator.userAgent) && !window.opera;       
	function fileChange(target) {     
		var fileSize = 0;        
		if (isIE && !target.files) {    
			var filePath = target.value;    
			var fileSystem = new ActiveXObject("Scripting.FileSystemObject");       
			var file = fileSystem.GetFile (filePath);    
			fileSize = file.Size;   
		} else {   
			fileSize = target.files[0].size;    
		}  
		var size = fileSize / 1024;   
		if(size>10000){ 
			alert("附件不能大于10M");             
		}    
	}  
</script> 
</head> 
<body> 
<input type="file" name="contractFileName"  style="width: 500px;" onchange="fileChange(this);"/> 
</body> 
</html>