WP Insert Attachment

/**
 * Insert Attachment
 *
 * @access private
 * @since 0.1.3a
 * @see shortcode_insert_recipe
 *
 * @param string $file_handler
 * @param integer $post_id
 * @param boolean $set_thumbnails
 * @return integer|boolean
 */
private function insert_attachment( $file_handler, $post_id, $set_thumbnails = true )
{
    // check to make sure its a successful upload
    if ( UPLOAD_ERR_OK !== $_FILES[ $file_handler ]['error']  ) {
        __return_false();
    }

    // Include required files from wp-admin to make this work
    require_once( ABSPATH . "wp-admin" . '/includes/image.php' );
    require_once( ABSPATH . "wp-admin" . '/includes/file.php' );
    require_once( ABSPATH . "wp-admin" . '/includes/media.php' );

    $attachment_id = media_handle_upload( $file_handler, $post_id );

    // Check if thumbnails should be generated
    if ( $set_thumbnails )
    {
        update_post_meta( $post_id, '_thumbnail_id', $attachment_id );
    }

    return $attachment_id;
}