When rmail-edit-current-message gets a mime message which is too complex
for it to handle (i.e., anything but one "text/plain" part), it just
gives you the raw message to edit.  If you do so, rmail-cease-edit then
applies the transfer-encoding even though the message wasn't decoded,
and so on.  It also in some circumstances it also moves the marker for
the beginning of the following message.

The attached patch modifies rmail-cease-edit to check the
rmail-old-mime-state flag set by rmail-edit-current-message when it is
editing the raw message.  If set, rmail-cease-edit inserts the
already-encoded body without further processing.  It also deletes the
old body after inserting the new, rather than before, to avoid moving
the next message's marker.

                                        Ken

In GNU Emacs 25.2.2 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
 of 2017-05-29 built on olum.org
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:	Ubuntu 16.04.3 LTS

Configured features:
XPM JPEG TIFF GIF PNG SOUND NOTIFY FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS
LUCID X11