JSFiddle - React, Tailwind, and code Playground

HTML

<input id="files" type="file" />

JavaScript

function handleFileSelect(evt) {
    var validExtensions = ["jpg", "jpeg", "gif", "png", "mp3", "mp4"];
    var files = evt.target.files[0];
    var ext = files.name.split('.').pop();
    if (validExtensions.indexOf(ext) > -1) {
        alert("Valid");
    } else {
        alert("Invalid");
    }
}
document.getElementById('files').addEventListener('change', handleFileSelect, false);