Instead of =LEFT(CELL("filename"),FIND("[",CELL("filename"))-1) return path of current worksheet.
Also: Make arguments universal between different language packs
I totally agree that a better way of getting the path and filename in the CELL function is needed.
This would also allow the exact filename to be found more easily; eg by using SUBSTITUTE(CELL("Filename"),CELL("Path","")). The current way of finding a file's name is cumbersome.
Of course, being able to do it direct would be even better; eg CELL("Filename_short").
Kenneth Barber commented