.fmt files missing in speedbar

[jm]

In emacs -Q I evaluate:

    (add-to-list 'auto-mode-alist '("\\.fmt\\'" . html-mode))
    (setq
     speedbar-directory-unshown-regexp "^\(\.\.*$\)\'"
     speedbar-show-unknown-files t)

but browsing a directory in the speedbar containing ‘file.fmt’ files
appears empty. Changing the extension to ‘.FMT’ makes it appear. In fact
I cannot find any other extension that causes a file to not appear; e.g.
‘.file.ext’, ‘file.bonkers’, ‘file.♑’, etc. all show up.

What am I missing here?

In GNU Emacs 30.0.50 (build 3, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.17.8) of 2023-06-21 built on localhost
Repository revision: 72f1c12e58ec2654779506480774e627d46d3693
Repository branch: master
System Description: Fedora Linux 38 (Workstation Edition)

Configured using:
 'configure --with-native-compilation --with-pgtk'

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

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