Display all printable multipart/mixed mime parts

[meskio]

Describe the bug
Is not uncommon that an email contains multiple printable mimeparts in a multipart/mixed. Currently only the first printable part gets displayed and the others are ignored. You need to use togglemimetree or something similar to see that there is something more there. Alot used to display all the parts, but somehow recently it has stopped doing so.

Some common cases I know of multiple multipart/mixed printable parts:

• mailman mailing lists. Is common for mailman mailing list to have a multipart/mixed with:
  1. the body of the original email
  2. a text/plain of a footer with Content-Disposition: inline
• schleuder mailing lists. Inside the encrypted email there is a multipart/mixed with two parts:
  1. a text/plain (without any Content-Disposition) with the original headers of the email. As schleuder replaces the unencrypted headers of the email to don't leak the sender.
  2. the original body of the email.

In the case of mailman not seeing the footer is not so bad. But in schleuder mailing lists we only see the original headers and not the body of the email.

Software Versions

• Python version: 3.8.3
• Notmuch version: 0.29.1
• Alot version: git master 2348014

To Reproduce
Steps to reproduce the behaviour:

1. open a thread from a mailman/schleuder mailing list
2. see that the displayed content is not all that you can see with togglemimeparts

[meskio]

I see the change was introduced by #1421, by f99ba2b. Before (in extract_body) there was some walk around the parts collecting a list of parts to display, after that change only one part will be selected and displayed.

@pazz is this change intentional? Should I give it a try to bring the functionality back so alot displays all multipart/mixed parts?

[pazz]

This was not a change I intended but also nothing I can't live with. Basically I'm very happy to delegate the extraction of a message "body text" to the email library instead of hacking or own methods.
That said, you're welcome to see if the email lib based thing can be tweaked to do what you want and make it configurable.