SUBSTRING(FileName,LEN(FileName) - FINDSTRING(REVERSE(FileName),"\\",1) + 2,LEN(RIGHT(FileName,FINDSTRING(REVERSE(FileName),"\\",1) - 1)))
All around the world..
SUBSTRING(FileName,LEN(FileName) - FINDSTRING(REVERSE(FileName),"\\",1) + 2,LEN(RIGHT(FileName,FINDSTRING(REVERSE(FileName),"\\",1) - 1)))