http://www.w3.org/TR/DOM-Level-2-HTML/html.html#ID-49531485说：
value of type DOMString
When the type attribute of the element has the value “text”,“file” or “password”,this represents the current contents of the corresponding form control,in an interactive user agent.
<html>
<head><title>...</title>
<script type="text/javascript">
function foo() {
var f = document.getElementById("f1");
alert( ""==f.value ? "nothing selected" : "file selected");
}
</script>
</head>
<body>
<form>
<div>
<input id ="f1" type="file" name="x" />
</div>
</form>
<button onclick="foo()">click</button>
</body>
</html>
