选择文件时，选多个文件

  CALL METHOD CL_GUI_FRONTEND_SERVICES=>FILE_OPEN_DIALOG
    EXPORTING
*     WINDOW_TITLE            =
*     DEFAULT_EXTENSION       =
*     DEFAULT_FILENAME        =
*     FILE_FILTER             =
*     WITH_ENCODING           =
*     INITIAL_DIRECTORY       =
      MULTISELECTION          = 'X'
    CHANGING
      FILE_TABLE              = PT_FILE_TABLE[]
      RC                      = L_RC
*     USER_ACTION             =
*     FILE_ENCODING           =
    EXCEPTIONS
      FILE_OPEN_DIALOG_FAILED = 1
      CNTL_ERROR              = 2
      ERROR_NO_GUI            = 3
      NOT_SUPPORTED_BY_GUI    = 4
      OTHERS                  = 5.