How can you attach a file to a mail using Symfony mailer?

I want to attach a file to a mail from an input file in my form. Here is how I build the mail :

$mailManager = \Drupal::service('plugin.manager.mail');
$params['title'] = 'title test';
$params['message'] = 'my message example';
$form_file = $form_state->getValue('my_input_file');
if (isset($form_file[0]) && !empty($form_file[0])) {
      $file = File::load($form_file[0]);
      $params['attachments'][] = $file;
}
$to = '[email protected]';
$result = $mailManager->mail('my_module', 'my_key', $to, \Drupal::currentUser()->getPreferredLangcode(), $params, NULL, TRUE);

I'm also using an EmailBuilder for rendering the mail :

/**
 * Defines the Email Builder plug-in for my_module module.
 *
 * @EmailBuilder(
 *   id = "my_module",
 *   sub_types = { },
 * )
 */
class DemandeCreationEmailBuilder extends EmailProcessorBase
{

  /**
   * {@inheritdoc}
   */
  public function preRender(EmailInterface $email)
  {
    $email->setSubject($email->getParam('title'));
    $email->setBody(array(
        'site' => \Drupal::config('system.site')->get('name'),
        'message' => $email->getParam('message'),
        'attachments' => $email->getParam('attachments'), // I tried this but it's not working
      )
    );
  }

}

The data from message is correctly included in the mail, but the file is not.

I'm using the Symfony Mailer module (version : 1.0.0-alpha7) to send mails. My version of Drupal is 9.

Does anyone know how to solve this problem please ?

I haven't used that module yet, but I think its

$file = File::load($form_file[0]);
$params['attachments'][] = [
  'filepath' => $file->createFileUrl(FALSE),
  'filename' => $file->getFilename(),
  'filemime' => $file->getMimeType()
];