Find the File .EXT on the end of a File Name after last period

FileExtensionFromName.clw

  MAP
FileExtension  PROCEDURE(*STRING FN),STRING
    MODULE('RTL')
LenFastClip    PROCEDURE(CONST *STRING Text2Measure),LONG,NAME('Cla$FASTCLIP')
    END
  END
  
!--------------------------------------
FileExtension PROCEDURE(*STRING FN)!,STRING
L   USHORT,AUTO
Ext STRING(24),AUTO
    CODE
    L=INSTRING('.',FN,-1,SIZE(FN)) !Last Period
    IF L THEN
       Ext=SUB(FN,L+1,24)
!      L=LEN(CLIP(Ext))
       L=LenFastClip(Ext)
       IF L<=3 AND L THEN 
          RETURN UPPER(Ext[1 : L]) ! 3 Char .Ext .EXT
       ELSIF L THEN  !Period alone should not happen
          Ext[1]=UPPER(Ext[1])     ! Upper letter 1
          RETURN Ext[1 : L]
       END
    END    
    RETURN ''
  
!--------------------------------------
! Less Code - returns 1 blank if the impossible happens, a file with '.' and no EXT
!--------------------------------------
FileExtension PROCEDURE(*STRING FN)!,STRING
L   USHORT,AUTO
Ext STRING(24),AUTO
    CODE    
    L=INSTRING('.',FN,-1,SIZE(FN))
    IF ~L THEN RETURN ''.
    Ext=SUB(FN,L+1,24)
!    L=LEN(CLIP(Ext))
    L=LenFastClip(Ext)
    IF L<=3 AND L THEN RETURN UPPER(Ext[1 : L]). ! 3 Char UPPER
    IF ~L THEN L=1.       ! Period alone should never happen, if it does let's not GPF
    Ext[1]=UPPER(Ext[1])  ! Upper letter 1
    RETURN Ext[1 : L]