[Exim] forwardfile director and headers_add

I use this director for expanding some static lists,
when this director is called I would like an additional
header added. However the headers_add seems to be ignored.

fixlists:
driver = forwardfile
domains = lists.uib.no
file = /local/exim/lists/fix/${local_part}
user = exim
group = exim
no_check_local_user
errors_to = listmaster@???
headers_add = "X-someheader:"

Am I missing something or is it not possible to
add headers using the forwardfile director?

hmk