<!DOCTYPE html>
<html>
<head>
<title>Title of the Document</title>
</head>
<body>
<h2>Select a file and click on the button to check the file extension.</h2>
<div>
<input type="file" id="chooseFile" />
<input type="button" value="Check Extension" onclick="checkFileExtension();" />
</div>
<h3>
The file extension is a:
<p class="output"></p>
</h3>
<script>
function checkFileExtension() {
fileName = document.querySelector('#chooseFile').value;
extension = fileName.split('.').pop();
document.querySelector('.output').textContent = extension;
};
</script>
</body>
</html>