ORACLE SQL | Modificar datos en ORDEN POR

Cambia tu ORDEN POR para ordenarlo por un numérico:

 ORDER BY regexp_replace(FILE_NAME, '_m', 1),
          regexp_replace(FILE_NAME, '_l', 2),
          regexp_replace(FILE_NAME, '_r', 3);

por ejemplo

WITH t
  AS (SELECT '12345_l.tif' AS file_name FROM dual
      UNION
      SELECT '12345_m.tif' FROM dual
      UNION
      SELECT '12345_r.tif' FROM dual
      UNION
      SELECT '12345_x.tif' FROM dual
      UNION
      SELECT '12345_y.tif' FROM dual)
SELECT file_name
  FROM t
 ORDER BY regexp_replace(FILE_NAME, '_m', 1),
          regexp_replace(FILE_NAME, '_l', 2),
          regexp_replace(FILE_NAME, '_r', 3);

Da:

==============
12345_m.tif
12345_l.tif
12345_r.tif
12345_x.tif
12345_y.tif

Espero que ayude...

Alternativamente, podría usar:

 ORDER BY (CASE SUBSTR(file_name, INSTR(file_name, '_')+1, 1)
             WHEN 'm' THEN 1
             WHEN 'l' THEN 2
             WHEN 'r' THEN 3
             ELSE 4
            END) ASC;

POR EJEMPLO:

WITH t
  AS (SELECT '12345_l.tif' AS file_name FROM dual
      UNION
      SELECT '12345_y.tif' FROM dual
      UNION
      SELECT '12345_r.tif' FROM dual
      UNION
      SELECT '12345_x.tif' FROM dual
      UNION
      SELECT '12345_m.tif' FROM dual)
SELECT file_name
  FROM t
 ORDER BY (CASE SUBSTR(file_name, INSTR(file_name, '_')+1, 1)
             WHEN 'm' THEN 1
             WHEN 'l' THEN 2
             WHEN 'r' THEN 3
             ELSE 4
            END) ASC;

Da:

12345_m.tif
12345_l.tif
12345_r.tif
12345_x.tif
12345_y.tif