Archive Query

gistfile1.aw

<?php

add_filter( 'pre_get_posts', 'be_archive_query' );
/**
 * Archive Query
 *
 * Sets all archives to 27 per page
 * @since 1.0.0
 * @link http://www.billerickson.net/customize-the-wordpress-query/
 *
 * @param object $query
 */
function be_archive_query( $query ) {
	if( $query->is_main_query() && $query->is_archive() ) {
		$query->set( 'posts_per_page', 27 );
	}
}

The documentation is out of sync with the code (nothing to do with attachments).
Also, the @SInCE should go under the long description (bit about 27 per page).

Updated