emacs:show: separate out handling of application/octet-stream

emacs:show: separate out handling of application/octet-stream

Currently mime parts are basically handled based on their mime-type
with the exception of application/octet-stream parts. Deal with these
parts at the top level (notmuch-show-insert-bodypart).

This is needed later in the series as we need to put in a part button
for each part (which means knowing its mime type) while deferring the
actual insertion of the part.