How to modify the content of an email

[alejandro.valdez@gmail.com]

Hello, I'm trying to make a python script that take an email in (raw)
text format, and add a footer to the text (or html) body of the email.

I'm aware of the email and email.mime modules, but I can't figure out
how to identify 'the main text (or html) content' from the email, and
how to be sure that I don't incorrectly identify a txt (or html)
attach as the main content of the email.

By 'main text (or html) content' I mean the text (or html) that is
showed by a Mail User Agent when it display the email to the
recipient.

Thanks in advance.