javascript监测(控制 )上传文件大小

<script language="javascript">
var oForm = new Object();
var oImage = new Image();
oImage.attachEvent('onload',getSize);
oImage.attachEvent('onerror',doWithError);
function checkImage(obj){
oForm = obj;
oImage.src = oForm.oFile.value;
}
function getSize(){
var oImgSize = Math.floor(oImage.fileSize/1024);
if(oImgSize>200){
window.alert('不允许上传大于200KB的图片！');
oForm.reset();
}
}
function doWithError(){
window.alert('出现错误：请重新选择图片上传！');
oForm.reset();
}
</script>

<form name="data_form" method="post" enctype="multipart/form-data">
<input type="file" name="oFile" onpropertychange="javascript:checkImage(document.data_form);">
</form>