This will return the correct results but only if you change your filename data type to varchar:
SELECT CHARINDEX(@filename,@FILEPATH), PATINDEX('%' + REPLACE( @filename, '[', '[[]') + '%',@FILEPATH)
This should show you the difference
DECLARE @filename varCHAR(64)
SET @filename = 'ASTERIX [Converted].eps.ai'
SELECT '%' + @filename + '%'
GO
DECLARE @filename CHAR(64)
SET @filename = 'ASTERIX [Converted].eps.ai'
SELECT '%' + @filename + '%'
GO