#ajax on a managed_file custom form field is not working
I don't think you can attach multiple ajax callbacks to a form element. But you could override the callback by extending the core element:
/src/Element/MyManagedFile.php
&lt;?php

namespace Drupal\mymodule\Element;

use Drupal\file\Element\ManagedFile;
use Drupal\Core\Ajax\AlertCommand;
use Drupal\Core\Form\FormStateInterface;
use Symfony\Component\HttpFoundation\Request;

/**
 * My Managed File Element.
 *
 * @FormElement(&quot;my_managed_file&quot;)
 */
class MyManagedFile extends ManagedFile {

 /**
 * {@inheritdoc}
 */
 public static function uploadAjaxCallback(&amp;$form, FormStateInterface &amp;$form_state, Request $request) {
    $response = parent::uploadAjaxCallback($form, $form_state, $request);
    $response-&gt;addCommand(new AlertCommand('Hello!!!'));
    return $response;
  }

}

And then use it in the custom form:
 $form['file'] = array(
      '#title' =&gt; t('Image File'),
      '#type' =&gt; 'my_managed_file',