Hello,

While experimenting with code to create image from data, I encountered
an issue with the regexp in `image-type-header-regexps' used to
auto-detect PBM image type from the first bytes of image data. That is:

"\\`P[1-6]\\(?:\
\\(?:\\(?:#[^\r\n]*[\r\n]\\)*[[:space:]]\\)+\
\\(?:\\(?:#[^\r\n]*[\r\n]\\)*[0-9]\\)+\
\\)\\{2\\}"

Here is a simple recipe to illustrate the issue:

In *scratch* buffer eval:
-------------------------
;; Get content of a pbm file.
(setq test-data
       (with-current-buffer
           (find-file-noselect "[YourEmacsPath]/etc/images/splash.pbm")
         (prog1 (buffer-substring-no-properties (point-min) (point-max))
           (kill-buffer (current-buffer)))))

;; Check string data fail for pbm image-type!
(image-type-from-data test-data)
>>> nil
;; With a temp buffer current, the same test works!
(with-temp-buffer
  (image-type-from-data test-data))
>>> pbm
-------------------------

After further digging, I found that the problem might be due to the use
of the [:space:] character class whose meaning, according to the manual,
depends on the syntax of whitespace characters setup in current buffer.
So, using discrete values in place of syntax class seems to solve the
issue:

(setcar (nth 1 image-type-header-regexps)
         "\\`P[1-6]\\(?:\
\\(?:\\(?:#[^\r\n]*[\r\n]\\)*[ \t\r\n]\\)+\
\\(?:\\(?:#[^\r\n]*[\r\n]\\)*[0-9]\\)+\
\\)\\{2\\}")

(image-type-from-data test-data)
>>> pbm

I attached a patch proposal.
Hope it will help.
Regards


In GNU Emacs 30.0.50 (build 3, x86_64-pc-linux-gnu, GTK+ Version
  3.24.38, cairo version 1.17.8) of 2023-08-23
Repository revision: 26ca3e84e167f975afb4e9e9a838935bfe4a19a7
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 38 (KDE Plasma)

Configured using:
  'configure --with-x-toolkit=gtk3
  --with-native-compilation=no
  PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/usr/lib/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

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