[Exim] How to get filename in an address_file pipe transport…

Suppose I have the following sort of setup:

    # transports
    my_save_file:
    driver = pipe
    command = "............"

    # directors
    filter:
    driver = forwardfile
    filter
        file_transport = my_save_file

and a filter triggered by a message through that director contains

    save foo

How can I get at that filename (foo) in the command run by the
my_save_file transport? There doesn't seem to be a string variable
containing the filename and it looks as though the filename may only
get passed behind the scenes to appendfile transports.

--Malcolm

--
Malcolm Beattie <mbeattie@???>
Unix Systems Programmer
Oxford University Computing Services