Something

$stmt = '
SELECT 
	`df`.`id`, 
	`df`.`full_name`, 
	`df`.`filesize`, 
	`df`.`hits`, 
	ROUND(SUM(`dr`.`rating`) / COUNT(`dr`.`id`), 2) AS `rating` 
FROM 
	`downloads_files` AS `df` 
LEFT JOIN 
	`downloads_ratings` AS `dr` ON `dr`.`file_id` = `df`.`id`
WHERE 
	' . $this->getCriterias() . ' 
GROUP BY 
	`df`.`id`
ORDER BY 
	`' . $sorting['by'] . '` ' . $sorting['type'] . ', `df`.`filename` ASC
LIMIT 
	' . ( int )$offset * $this->files_to_fetch . ', ' . $this->files_to_fetch;