function beforeSubmit(){var fname = document.getElementById("ifile").value;// check if fname has the desired extensionif (fname hasDesiredExtension) {return true;} else {return false;}}
<!-- Right approach: Use both file extensions and their corresponding MIME-types. --><!-- (IE 10+, Edge (EdgeHTML), Edge (Chromium), Chrome, Firefox) --><input type="file"accept=".xls,.xlsx, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet,application/vnd.ms-excel" />
File API is very well supported for modern browsers. By combining this with the accept attribute you can easily filter what the local user can select in an upload and provide useful feedback. If you are uploading the file, you should still check and validate the file type on your backend.