0. Save the attached file e.g. as "~/test".
1. emacs -Q
2. M-x gnus, type `y' at the prompt.
3. In the *Group* buffer type `G f test RET'.
4. Type RET to enter Gnus Summary mode showing the file as mail.
5. In the Summary buffer type RET to open the mail.
6. In the Summary buffer type `2 b' to open the buttonized
   message/rfc822 attachment display in the Article buffer.
=> The attachment is displayed as the untreated raw text of the file
(i.e. with all headers, quoted-printable-encoded) instead of as a normal
email, as it is when repeating the recipe in emacs-27.

This bug is due to this change (reverting it restores the emacs-27
display):

commit 748bf7b93edcfd1707fa3f593a330979e280eb49
Author:     Lars Ingebrigtsen <larsi@gnus.org>
Commit:     Lars Ingebrigtsen <larsi@gnus.org>
CommitDate: Tue Jul 6 16:34:24 2021 +0200

    Allow using `mm-inline-message' from other mail clients than Gnus

    * lisp/gnus/mm-view.el (mm-inline-message-prepare-function): New
    variable (bug#49380).
    (mm-inline-message): Use it to separate out the Gnus-specific code.
    * lisp/gnus/gnus-art.el (gnus-mime-display-single): ... which has
    been moved here.

When debugging I see that mm-inline-message-prepare-function is nil in
mm-inline-message.  AFAICT this is because at step 6 in the recipe,
gnus-mime-display-part is not called, and hence gnus-mime-display-single
is not called.  But I haven't figured out why gnus-mime-display-part is
not called.


In GNU Emacs 28.0.60 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.29, cairo version 1.17.4)
 of 2021-10-16 built on strobelfs
Repository revision: ac6ac76e3ae5ca96607ac7eba4a3ccf146fc8815
Repository branch: emacs-28
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Linux From Scratch 10.2-rc1

Configured using:
 'configure --with-native-compilation 'CFLAGS=-Og -g3''

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LCMS2 LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG
RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM
GTK3 ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix