alter email body

Hi,
i am looking for an way to add an mulitpart/alternative disclaimer to an
email.
A simple $transaction->body_write($my_mime) did not work.

Here is what i coded so far:

use MIME::Entity;
use MIME::Parser;

sub register {
        my ($self, $qp) = @_;
        $self->register_hook("data_post", "addsig");
}

sub addsig {
        my ($self, $transaction) = @_;

        $transaction->body_resetpos;
        my $ent = MIME::Entity->build('Type' =>
"multipart/alternative",
                                );
        $ent->attach(Path => "/etc/qpsmtpd/sigs/disc.txt");
        $ent->attach(Path => "/etc/qpsmtpd/sigs/disc.html");
        $transaction->body_write($ent->as_string);
        return (DECLINED);
}

As far as i can see, nothing hapened. The email goes through the plugin
without being altered.

Does someone of you have a suggestion?

regards