bug#34346: 26.1; Big hang when opening text (racket) files with .art extension (image-mode)

[Pierre Neidhardt]

Tested with emacs -Q: opening a .art file
(e.g. 
https://raw.githubusercontent.com/euhmeuh/rilouw.eu/master/articles/hello.art)
will hang Emacs for about a minute (on a rather powerful computer).  CPU
is about 0% but disk I/O oscillates between 80MB/s-400MB/s (your mileage
may vary).

It seems to be because Emacs tries to open the .art file in image-mode.
The regexp seems to be added by imagemagick-register-types.

Removing .art from auto-mode-alist works.  The following workaround will
remove the complete regexp, that is, all extensions added by
imagemagick-register-types.

  (dolist (pair auto-mode-alist)
    (when (string-match (car pair) ".art")
      (delete pair auto-mode-alist)))





In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.0)
 of 2018-11-14 built on localhost
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000

Configured using:
 'configure
 
CONFIG_SHELL=/gnu/store/q19l04vd2za80mk1845pz7r8cz29qk43-bash-minimal-4.4.23/bin/bash
 SHELL=/gnu/store/q19l04vd2za80mk1845pz7r8cz29qk43-bash-minimal-4.4.23/bin/bash
 --prefix=/gnu/store/vsiqlxhj7lnydhhi85jc1pg0xzhcfdny-emacs-26.1
 --enable-fast-install'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS GSETTINGS NOTIFY ACL
GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 THREADS

Important settings:
  value of $LANG: en_US.utf8
  locale-coding-system: utf-8-unix

Major mode: mu4e:view
<#secure method=pgpmime mode=sign>

--
Pierre Neidhardt
https://ambrevar.xyz/