On Mon, 27 Mar 2017, Juri Linkov wrote:

>> emacs -Q /tmp
>>
>> (require 'dired-x)
>> (require 'ert)
>> ;; Evaluate following sexp:
>> (let ((dirs '("Public" "Music"))
>>       (files '(".bashrc" "bar.c" "foo.c"))
>>       (dir (make-temp-file "Bug25942" 'dir)))
>>   (unwind-protect
>>       (progn
>>         (dolist (d dirs)
>>           (make-directory (expand-file-name d dir)))
>>         (dolist (f files)
>>           (write-region nil nil (expand-file-name f dir)))
>>         (dired dir)
>>         (dired-mark-extension "c")
>>         (should (= 2 (length (dired-get-marked-files)))))
>>     (delete-directory dir 'recursive)))
>>
>> IMO, in this example looks better if just those files ending with
>> '.c' are marked i.e., to not mark 'Public' or 'Music'.
>
> Is this a backward-compatible change?
Yes, it is; but it makes a better global picture when we consider
this function together with `file-name-extension' and
`file-name-sans-extension' (More below).

> If not, then maybe better
> prepend ‘.’ only to the default value for 
> interactive uses?  So users
> will get the right result when using ‘M-x dired-mark-extension RET RET’.
> But for the programmatic uses it's the responsibility of the caller to
> supply the right suffix in ‘(dired-mark-extension ".c")’
The problem is that this command has a concept of 'extension' different 
than the two other functions mentioned above.
[Using same names than in my code snippet]:

(file-name-extension "Music")
=> nil
(equal "Music" (file-name-sans-extension "Music"))
=> t

(file-name-extension "foo.c")
=> "c"
;; Note: this command consider extension "c" (without period).

(dired-mark-extension (file-name-extension "foo.c"))
;; This marks all the files, i.e., also marks "Music" and "Public".

IMHO the correct behaviour for
(dired-mark-extension (file-name-extension "foo.c"))
would be to mark those files satisfying
(lambda (f) (equal (file-name-extension f) (file-name-extension "foo.c")))