How to create a 'text version' of an email broadcast?

Whenever we send out emails with images, etc., and we check the score from Spam Assassin, we keep on getting this issue:

Message only has text/html MIME parts
You should also include a text version of your message (text/plain)

Seems like it wants us to provide a text version of the email, but I don’t see any feature that allows us to do that in the email broadcast. I saw a couple of topics answered regarding this, but they don’t seem to work anymore. Is there a way to fix this or is this an existing issue up to now?