JSFiddle - React, Tailwind, and code Playground

HTML

<input accept=".cer,.pem,.der" id="certRegField" name="user_cert" type="file">

JavaScript

$(function(){
  $('#certRegField').change(function(){
    var url = $(this).val();
    var ext = url.substring(url.lastIndexOf('.') + 1).toLowerCase();
    if (!(ext == "cer" || ext == "pem" || ext == "der")){
    	document.getElementById("certRegField").value = "";
    	alert("Unaccepted file format, try again.");
    }
  });
});