emacs: bugfix attachment content-type as mime-type handling

emacs: bugfix attachment content-type as mime-type handling

Notmuch puts attachments in as declared content-type except when the
content-type is application/octet-stream it tries to guess the type
from the filename/extension. This means that viewing a pdf (for
example) which is sent as application/octet-strem invokes the pdf
viewer rather than just offering to save the part.

Recent changes to the attachment handling (commit 1546387d) changed
(broke) this. This patch stores the calculated mime-type with the part
and changes the attachment part handlers can use it instead.