[Exim] The easiest and clean way to change message body code…

Dear sirs,
all I want is just codepage translation in both inbound and outbound mail.
Filter program already created. Translating standard input to standard
output.
What are the ways in Exim to invoke such program?
Please dont help only with words about pipes, directors and routers, I read
all documentation!
And still confused...
If possible small real example.

Thanks, Regards,
Oleg Chibarov