Subject: bug#13006: recognizing compressed files with arbitrary names

From: Glenn Morris <[email-masked]>
Date: Tue, 27 Nov 2012 13:41:11 -0500

Andreas Schwab wrote:

> This is a regression from Emacs 23.  It should disregard the backup
> suffix when matching the extension.

Presumably the 2010-12-08 change to jka-compr-compression-info-list.
(There is a comment above jka-compr-compression-info-list which was
ignored and no longer applies. Also the :version was not bumped.)
Anyway, this seems to fix the immediate problem:

*** lisp/jka-cmpr-hook.el 2012-01-29 17:41:43 +0000
--- lisp/jka-cmpr-hook.el 2012-11-27 18:35:37 +0000
***************
*** 109,114 ****
--- 109,115 ----
    "Return information about the compression scheme of FILENAME.
  The determination as to which compression scheme, if any, to use is
  based on the filename itself and `jka-compr-compression-info-list'."
+   (setq filename (file-name-sans-versions filename))
    (catch 'compression-info
      (let ((case-fold-search nil))
        (dolist (x jka-compr-compression-info-list)

There's a separate question of handling arbitrarily named compressed
files via magic-fallback-mode-alist or somesuch.