mu4e-view: add mu4e-view-jump-to-mime-part

This allows for jumping to the MIME-part by number; a thin wrapper over
the Gnus functionality.

mu4e/mu4e-view.el

@@ -853,6 +853,11 @@ Note that for some messages, this can trigger high CPU load."
         (list handle attendee))
   handle-attendee))
 
+(defun mu4e-view-jump-to-mime-part (number)
+  "Jump to MIME-part with NUMBER."
+  (interactive "P")
+  (call-interactively #'gnus-article-jump-to-part number))
+
 (defun mu4e--view-mode-p ()
   "Is the buffer in mu4e-view-mode or one of its descendants?"
   (or (eq major-mode 'mu4e-view-mode)
@@ -925,6 +930,7 @@ This is useful for advising some Gnus-functionality that does not work in mu4e."
     (define-key map "a" #'mu4e-view-action)
     (define-key map "A" #'mu4e-view-mime-part-action)
     (define-key map "e" #'mu4e-view-save-attachments)
+    (define-key map "J" #'mu4e-view-jump-to-mime-part)
 
     ;; change the number of headers
     (define-key map (kbd "C-+") #'mu4e-headers-split-view-grow)

mu4e/mu4e.texi

@@ -1420,6 +1420,7 @@ e            extract (save) one or more attachments (asks for numbers)
 (or: <mouse-2> or S-RET with point on attachment)
 a            execute some custom action on the message
 A            execute some custom action on the message's MIME-parts
+J            jump to MIME part by its number
 
 misc
 ----
@@ -1475,6 +1476,10 @@ even clearer by indicating them in the message itself, using:
 (setq gnus-unbuttonized-mime-types nil)
 @end lisp
 
+Note that you can jump to the relevant MIME-parts using
+@code{mu4e-view-jump-to-mime-part}, which ask for the number or uses the
+prefix-argument.
+
 @subsection Inline images
 When you run Emacs in graphical mode, by default images attached to
 messages are shown inline in the message view buffer.