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<!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.slice((fileName.lastIndexOf(".") - 1 >>> 0) + 2); document.querySelector('.output').textContent = extension; }; </script> </body> </html>