Hi Monica Hu,
Remove file type and try
CALL FUNCTION 'GUI_DOWNLOAD'
EXPORTING
filename = 'D:/sflight.xls'
TABLES
data_tab = it_header.
CALL FUNCTION 'GUI_DOWNLOAD'
EXPORTING
filename = 'D:/sflight.xls'
* filetype = 'ASC'
append = 'X'
write_field_separator = 'X'
TABLES
data_tab = it_sflight.