Ошибка при загрузке изображения в WordPress с помощью media_handle_upload

Question

Здравствуйте, я пытаюсь загрузить изображение из формы в WordPress, но продолжаю получать: Ошибка загрузки файла! без какой-либо подсказки, что я делаю неправильно:

require_once ($ _ SERVER ['DOCUMENT_ROOT']. '/wp-load.php');

  if($_FILES['trainee_image']['name']) {
  if(!$_FILES['trainee_image']['error']) {
  //validate the file
   $new_file_name = strtolower($_FILES['trainee_image']['tmp_name']);
  //can't be larger than 200 KB 
  if($_FILES['trainee_image']['size'] > (200000)) {
 //wp_die generates a visually appealing message element
  wp_die('Your file size is to large.');
  }
  else {
  //the file has passed the test
 //These files need to be included as dependencies when on the front    end.
  require_once( ABSPATH . 'wp-admin/includes/image.php' );
  require_once( ABSPATH . 'wp-admin/includes/file.php' );
  require_once( ABSPATH . 'wp-admin/includes/media.php' );

  // Let WordPress handle the upload.
 // Remember, 'upload' is the name of our file input in our form above.
 $file_id = media_handle_upload( 'trainee_image', 0 );

 if ( is_wp_error( $file_id ) ) {
 wp_die('Error loading file!');
} 
}
}
else {
//set that to be the returned message
wp_die('Error: '.$_FILES['trainee_image']['error']);
}
}