[exim] Attach gif or html to any outgoing Mail

Hello list,
i need a script which i can add to a transport in exim to add an
attachment to any mail which i send out over this server.
I know that its dangerous and perhaps not allowed to change mail-content
on a server, but it is a task of a customers chief.
The coworkers know that their mail will be changed with this attachment.
I searched by google and try out sending a mail via gmx, which do
something else with commercial news, by adding a section includes Boundary.
At this moment i have added a section to my configure-file which will
call an external script when the given domain matches.

dnslookup:
        driver = dnslookup
        domains = ! +localDomains
        transport = ${if eq {$sender_address_domain}{my.domain}\
        {remote_smtp_filter}{remote_smtp}}
        ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
        no_more

remote_smtp_filter:
driver = smtp
transport_filter = /external-script

Perhaps somebody has an idea how he script can look like
or has another solution for this task.

My exim is Version 4.12


Volker