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<?php

/**
 * Implements hook_mail().
 */
function MY_MODULE_mail($key, &$message, $params) {
  if ($key == 'MY_KEY') {
    $fid = 4; // For example
    $file = File::load($fid);
    $message['params']['attachments'][] = [
      'filename' => $file->getFilename(),
      'filepath' => $file->getFileUri(),
      'filemime' => $file->getMimeType(),
    ];
  }