You can only have 1 body, and a body consisting of both plain text and HTML is simply not possible. The first plain text attachment apparently ends up as body, so you could make that an explaining text.

If you want it all in the body, you could incorporate the plain text part as a <pre>-formatted part into the HTML and use that as body. I prefer plain text mails, therefore haven't done that yet, but it should work that way, too.