The following code for Gnus allows one to type <tt>C-u C-c C-a</tt> to attach file, auto-generating a detached signature for that same file and attaching it as well.

{{{
(require 'mml)

(defvar mml-signing-attachment nil)

(defun mml-sign-attached-file (file &optional type description disposition)
  (unless (or mml-signing-attachment
              (null current-prefix-arg))
    (let ((signature
           (expand-file-name (concat (file-name-nondirectory file) ".sig")
                             temporary-file-directory))
          (mml-signing-attachment t))
      (message "Signing %s..." file)
      (if t
          (call-process epg-gpg-program file nil nil
                        "--output" signature "--detach-sign" file)
        (with-temp-file signature
          (setq buffer-file-coding-system 'raw-text-unix)
          (call-process epg-gpg-program file t nil "--detach-sign")))
      (message "Signing %s...done" file)
      (mml-attach-file signature))))
}}}

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