bug#36401: 26.0.90; set-auto-mode uses case-insensitive match for magic-mode-alist

[Benjamin Riefenstahl]

My use case: I use "magic-mode-alist" to detect a binary file type for
which I have written a mode.  The magic at the beginning is "mTFf", so
it is simple ASCII, but unusually capitalized.  When I add this to
"magic-mode-alist", files and buffers that start with "mtff" are also
put in my binary file mode.

It turns out that "set-auto-mode" in lisp/files.el is the function that
uses magic-mode-alist, and it just calls "looking-at" to test the type.
Because "case-fold-search" is "t" by default, this catches files
starting with "mtff".

Is this intentional?  If so, the documentation for "magic-mode-alist"
should call this out, so users know they might need to work around it.
If not, I could propose a patch to fix it.

TIA, benny

-- 

In GNU Emacs 26.0.90 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.14.5)
 of 2017-10-18 built on riefenstahl-linux
Repository revision: b970a4a52a1c04afce4916ca2bf337e2b36d26c4
Windowing system distributor 'The X.Org Foundation', version 11.0.11604000
System Description:     Debian GNU/Linux 8.11 (jessie)

Recent messages:
Mark set
Saving file /home/riefenstahl/.emacs.cfg/local.riefenstahl-linux.el...
Wrote /home/riefenstahl/.emacs.cfg/local.riefenstahl-linux.el
(No changes need to be saved)
Auto-saving...done
Saving file /home/riefenstahl/.emacs.cfg/local.riefenstahl-linux.el...
Wrote /home/riefenstahl/.emacs.cfg/local.riefenstahl-linux.el
(No changes need to be saved) [6 times]
Making completion list...
Type "q" in help window to restore its previous buffer.

Configured features:
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ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 LCMS2

Important settings:
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Major mode: Emacs-Lisp

Minor modes in effect:
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Load-path shadows:
~/site-lisp/css-mode hides 
/usr/local/share/emacs/26.0.90/lisp/textmodes/css-mode

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Memory information:
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