Keep Trailing Spaces with GUI_DOWNLOAD

I've read a number of threads dealing with keeping trailing white spaces in output files, but none seem to get this to work with GUI_DOWNLOAD.

I have the call

call function 'GUI_DOWNLOAD'
EXPORTING
filename = fname "u_p_outfile
filetype = 'ASC'
TRUNC_TRAILING_BLANKS = ' '
TABLES
data_tab = u_t_output

I have checked u_t_output and it has spaces at the end of the lines.

I opened the output file with a hex editor and after the last non space character on each line there is a 0D0A (aka carriage return + line feed).

I was under the impression that TRUNC_TRAILING_BLANKS would work, but apparently not.