JSFiddle - React, Tailwind, and code Playground

HTML

<form id="test" method="post" action="/echo/html/" enctype="multipart/form-data">
    <input type="hidden" value='{"success":true}' name="html" />
  <input name="file" id="file" type="file" multiple="multiple"/>
</form>

JavaScript

var form = document.querySelector('#test');
form.addEventListener('change', change);
function change(event){
  var iframe = document.createElement('iframe');
  iframe.name = 'iframex';
  form.target = iframe.name;
    iframe.style.width = 0;
    iframe.style.height = 0;
    document.body.appendChild(iframe);
  iframe.addEventListener('load', load);
  form.submit();
}
function load(event){
  alert(this.contentWindow.document.body.textContent);  
}