JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" enctype="" onsubmit="return validate()">
    <input type="file" id="file" />
    <input type="submit" />
</form>

JavaScript

function validate() {
    var filename=document.getElementById('file').value;
    var extension=filename.substr(filename.lastIndexOf('.')+1).toLowerCase();
    //alert(extension);
    if(extension=='jpg' || extension=='gif') {
        return true;
    } else {
        alert('Not Allowed Extension!');
        return false;
    }
}