SELECT 
  PI.product_id, 
  F1.id AS file_id, 
  F1.path AS file, 
  (
    SELECT 
      PI1.alt 
    FROM 
      `cscart_products__images` PI1 
    WHERE 
      PI1.original_file_id = F1.id
  ) AS file_alt, 
  F2.id AS big_file_id, 
  F2.path AS big_file, 
  (
    SELECT 
      PI2.alt 
    FROM 
      `cscart_products__images` PI2 
    WHERE 
      PI2.watermark_big_file_id = F2.id
  ) AS big_file_alt, 
  F3.id AS small_file_id, 
  F3.path AS small_file, 
  (
    SELECT 
      PI3.alt 
    FROM 
      `cscart_products__images` PI3 
    WHERE 
      PI3.watermark_small_file_id = F3.id
  ) AS small_file_alt 
FROM 
  `cscart_products__images` AS `PI` 
  LEFT JOIN `files` AS `F1` ON `F1`.`id` = `PI`.`original_file_id` 
  LEFT JOIN `files` AS `F2` ON `F2`.`id` = `PI`.`watermark_big_file_id` 
  LEFT JOIN `files` AS `F3` ON `F3`.`id` = `PI`.`watermark_small_file_id` 
WHERE 
  `PI`.`product_id` = 28921