Bug: archiver/msgbody can treat an attachment as the body

The msgbody function in archiver.py tries to extract a body from the message.
However it does not take account of the disposition of the body parts, so may use the contents of a file rather than the inline text.

Start in archiver.py at the msgbody function and inspect how it chooses content from message parts. Check how each part's disposition is represented, then verify that an attachment is not selected as the body while inline text remains eligible.

Written by the indexing model from the issue text.