I do not know about all OSes, but here is a proc I use to clear file attributes:

PROCEDURE ClearAttributes
   *
   LPARAMETER tcFileName
   IF VARTYPE(tcFileName) <> "C" OR EMPTY(tcFileName)
      RETURN .F.
   ENDIF
   LOCAL lcFile, lnRetVal
   lcFile = ALLTRIM(tcFileName)
   DECLARE SHORT SetFileAttributes IN KERNEL32 ;
      STRING @ lpFileName, ;
      INTEGER dwAttributes
   lnRetVal = SetFileAttributes(lcFile, 0) && Remove all attributes
   RETURN (lnRetVal > 0)
ENDPROC