Message not being sent in the body of the mail
You need to add a hook_mail code

As this line $mailManager-&gt;mail($module, $key, $to, $langcode, $params, NULL, $send); behind the scenes will invoke this hook_mail function. 

something like this:

/**
* Implements hook_mail().
*/
function &lt;module_name&gt;_mail($key, &amp;$message, $params) {

 $options = array(
   'langcode' =&gt; $message['langcode'],
 );

 switch ($key) {
   case 'uploaded files':
     $message['from'] = \Drupal::config('system.site')-&gt;get('mail');
     $message['subject'] = t('@title', array('@title' =&gt; $params['node_title']), $options);
     $message['body'][] = $params['message'];
     break;
 }
}