notmuch-mode: Emails with PDF attachments incorrectly tagged as text/plain expose a few issues

[Leo Gaspard]

Hello,

I have recently started conversing with someone whose email client
incorrectly tags PDF attachments as text/plain.

This triggered at least three bugs in what I assume to be notmuch:

- `w` on the notmuch-show window, to save the attachments, corrupts the
  PDF before writing it (checked by also saving the same attachment from
  thunderbird, the PDF opened cleanly)

- `r` still on the notmuch-show window, while it does properly open a
  reply email to the given email, also adds all the PDF's text at the
  end of the reply, resulting in jumbled text down there

- Trying to send said reply mail triggers a freeze of emacs before
  actually sending the email (which I, this time, had to `kill` twice so
  that it would accept to shutdown without me resorting to `kill -9`'ing
  it -- I seem to remember that, last time this happened to me, I just
  waited not that long and it eventually managed to send, though I think
  I still had to kill emacs after that)

- (I'm not sure this one isn't a message-mode bug, hence not counting it
  -- but just trying `emacs -Q [the offending file]` then
  `M-x message-mode` didn't reproduce, so I'm just writing it down here)
  Trying to edit the `#*message*-YYYYMMDD-HHMMSS#` file left behind by
  the previous emacs freeze is awfully slow

Anyway, as usual, thank you for all your work on notmuch!

Cheers,
  Leo