the whole expression returns 0....try it as follows:

DECLARE @filename CHAR(64)

DECLARE @FILEPATH VARCHAR(MAX)

SET @filename = 'ASTERIX [Converted].eps.ai'

SET @FILEPATH = ':Volumes:Art WIP: CHILDRENS:ASTERIX: ALBUMS USING NEW FRENCH ARTWORK:20 - Asterix in Corsica:9780752866444_Asterix Corsica PB Folder:Links:ASTERIX [Converted].eps.ai'

SELECT PATINDEX('%[[]' + @filename + '][]%',@FILEPATH)