Add post-save actions for Rmail attachments

* lisp/mail/rmailmm.el (rmail-mime-save-action): New user
option.
(rmail-mime-save): Use it.
* doc/emacs/rmail.texi (Rmail Display): Document it (bug#78971).

doc/emacs/rmail.texi

@@ -1229,6 +1229,27 @@ summarizes the part's index, size, and content type.  Depending on the
 content type, it may also contain one or more buttons; these perform
 actions such as saving the part into a file.
 
+@vindex rmail-mime-save-action
+  When you save a MIME attachment to a file, Rmail can perform follow-up
+actions according to the value of @code{rmail-mime-save-action}.  By
+default, this value is @code{nil}, which means Rmail does nothing.
+Other predefined values are:
+
+@table @code
+@item visit-file
+Visit the saved file in Emacs.
+
+@item visit-directory
+Visit the file's directory in Dired and move point to the file.
+
+@item open-external
+Open the file with an external program appropriate for its type.
+@end table
+
+You can also set @code{rmail-mime-save-action} to a custom function.
+This function will be called with the absolute filename of the saved
+attachment as its only argument.
+
 @table @kbd
 @findex rmail-mime-toggle-hidden
 @item @key{RET}

etc/NEWS

@@ -1344,6 +1344,13 @@ replies.
 'rmail-re-abbrevs'.  'rmail-re-abbrevs' is still honored if it was
 already set.
 
++++
+*** New user option 'rmail-mime-save-action'.
+This option specifies an action to take after saving a MIME attachment.
+Predefined values include visiting the file in Emacs, jumping to the
+file in Dired, or opening the file with an external program.  You can
+also provide a custom function.
+
 ** Message
 
 +++

lisp/mail/rmailmm.el

@@ -242,6 +242,27 @@ TRUNCATED is non-nil if the text of this entity was truncated."))
 
 ;;; Buttons
 
+(defcustom rmail-mime-save-action nil
+  "Action to perform after saving a MIME attachment in Rmail.
+The value can be one of the following:
+
+- nil: Do nothing (default).
+- `visit-file': Visit the saved file in Emacs.
+- `visit-directory': Visit the file's directory in Dired.
+- `open-external': Open the file with an external program.
+- A function: Call the function with the absolute filename as argument.
+
+Email attachments can be dangerous.  When this variable is set to one of
+the predefined actions, the user will be prompted to confirm the action
+before it is performed.  If you set this variable to a function, it will
+be called without confirmation.  Please exercise caution."
+  :type '(choice (const :tag "Do nothing" nil)
+                 (const :tag "Visit file in Emacs" visit-file)
+                 (const :tag "Visit directory in Dired" visit-directory)
+                 (const :tag "Open with external program" open-external)
+                 (function :tag "Custom function"))
+  :version "31.1")
+
 (defun rmail-mime-save (button)
   "Save the attachment using info in the BUTTON."
   (let* ((rmail-mime-mbox-buffer rmail-view-buffer)
@@ -282,7 +303,22 @@ TRUNCATED is non-nil if the text of this entity was truncated."))
 		 (ignore-errors (base64-decode-region (point-min) (point-max))))
 		((string= transfer-encoding "quoted-printable")
 		 (quoted-printable-decode-region (point-min) (point-max))))))
-      (write-region nil nil filename nil nil nil t))))
+      (write-region nil nil filename nil nil nil t))
+    (pcase rmail-mime-save-action
+      ('nil nil)
+      ('visit-file
+       (when (yes-or-no-p (format "Visit attachment `%s' in Emacs? "
+                                  (file-name-nondirectory filename)))
+         (find-file filename)))
+      ('visit-directory
+       (when (yes-or-no-p (format "Visit attachment `%s' in Dired? "
+                                  (file-name-nondirectory filename)))
+         (dired-jump nil filename)))
+      ('open-external
+       (when (yes-or-no-p (format "Open attachment `%s' with external program? "
+                                  (file-name-nondirectory filename)))
+         (shell-command-do-open (list filename))))
+      ((pred functionp) (funcall rmail-mime-save-action filename)))))
 
 (define-button-type 'rmail-mime-save 'action 'rmail-mime-save)