Web Guru...: Get file extension in Javascript

<html>
<head>
<script language="JavaScript">
function validate() {
var filename = document.getElementById("Upload_File").value;
var ext = getExt(filename);
if(ext == "pdf")
return true;
alert("Please upload PDF files only.");
return false;
}
function getExt(filename) {
var dot_pos = filename.lastIndexOf(".");
if(dot_pos == -1)
return "";
return filename.substr(dot_pos+1).toLowerCase();
}
</script>
</head>
<body>
<form action="test2.htm" method="post" enctype="multipart/form-data" onsubmit="return validate()">
<input type="file" id="Upload_File" />
<input type="submit" value="Upload" />
</form>
</body>
</html>

Web Guru...