gmail中写邮件时附件的选择和显示是怎么实现的？

<body>
<input type="file" id="upload" name="upload" />
<div id="display"></div>
<script type="text/javascript">
var obj=document.getElementById("upload");
obj.style.display="none";
obj.click();
var message=document.getElementById("display");
if(obj.value!="")
{
message.innerHTML+="<b>"+obj.value+"</b>";
}
</script>
</body>