Message not being sent in the body of the mail

Question

I am using the code below and the mail gets sent but without the body of the message, but the subject is correct am I sending the body in thee wrong parameter?

 function mailMessage($msg){
 $mailManager = \Drupal::service('plugin.manager.mail');
 $module = "mymodule";
 $key = 'uploaded files';
 //$to = \Drupal::currentUser()->getEmail();
 $to="[email protected]";
 $params['message'] = "THIS IS my body of the message";
 $params['node_title'] = "File Upload Report";
 $params['subject'] = "File Upload Report";
 $langcode ="en";
 $send = true;
 $result = $mailManager->mail($module, $key, $to, $langcode, $params, NULL, $send);
 if ($result['result'] !== true) {
   drupal_set_message(t('There was a problem sending your message and it was not sent.'), 'error');
 }
 else {
   drupal_set_message(t('Your message has been sent.'));
 }
}

Answer 1

The body of the message is set by the implementation of hook_mail() done from the module sending the message. The code invoking that hook is the following one. (See MailManager::doMail(), the method called from MailManager::mail().)

  // Bundle up the variables into a structured array for altering.
  $message = array(
    'id' => $module . '_' . $key,
    'module' => $module,
    'key' => $key,
    'to' => $to,
    'from' => $site_mail,
    'reply-to' => $reply,
    'langcode' => $langcode,
    'params' => $params,
    'send' => TRUE,
    'subject' => '',
    'body' => array(),
  );

  // Build the default headers.
  $headers = array(
    'MIME-Version' => '1.0',
    'Content-Type' => 'text/plain; charset=UTF-8; format=flowed; delsp=yes',
    'Content-Transfer-Encoding' => '8Bit',
    'X-Mailer' => 'Drupal',
  );
  // To prevent email from looking like spam, the addresses in the Sender and
  // Return-Path headers should have a domain authorized to use the
  // originating SMTP server.
  $headers['Sender'] = $headers['Return-Path'] = $site_mail;
  $headers['From'] = $site_config->get('name') . ' <' . $site_mail . '>';
  if ($reply) {
    $headers['Reply-to'] = $reply;
  }
  $message['headers'] = $headers;

  // Build the email (get subject and body, allow additional headers) by
  // invoking hook_mail() on this module. We cannot use
  // moduleHandler()->invoke() as we need to have $message by reference in
  // hook_mail().
  if (function_exists($function = $module . '_mail')) {
    $function($key, $message, $params);
  }

As you see, $message['body'] is set to an empty array before passing it to hook_mail(). If your module doesn't implement hook_mail(), $message['body'] will stay an empty array, if there aren't other module that implement hook_mail_alter().
The comment before the code invoking hook_mail() is also explicit about what expecting from that hook: get subject and body, allow additional headers.

As usual, that is the default implementation of the mail manager. It could be a different mail manager sets the message body to a more significant value, but a module should not depend on the mail manager doing it.

As side note, Drupal doesn't use $message['message'] as your code does.

Answer 2

You need to add a hook_mail code

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

something like this:

/**
* Implements hook_mail().
*/
function <module_name>_mail($key, &$message, $params) {

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

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