[RFC] Content-Description when naming MIME parts in Emacs mode

[W. Trevor King]

On the rss2email list, Victor Orlikowski pointed out [1] that a number
of MUAs don't use the Subject header of attached message/rfc822 parts
to label multipart/digest subparts [2].  Instead, notmuch and several
other MUAs use the filename parameter [3] as a content hint [4].
Using the filename parameter seems more sane than diving into the
message/rfc822 part header, but that's still not what the filename
parameter was designed for.  It makes more sense to me to use the
message/rfc822 part's Content-Description header [5,6], falling back
on the filename parameter if Content-Description isn't set.

It's pretty easy to patch notmuch-show-insert-bodypart to do this:

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index 1ac80ca..485c7d1 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -874,13 +874,16 @@ useful for quoting in replies)."
                         content-type))
          (nth (plist-get part :id))
          (beg (point))
+         (name (if (plist-get part :content-description)
+                   (plist-get part :content-description)
+                   (plist-get part :filename)))
          ;; Hide the part initially if HIDE is t.
          (show-part (not (equal hide t)))
          ;; We omit the part button for the first (or only) part if
          ;; this is text/plain, or HIDE is 'no-buttons.
          (button (unless (or (equal hide 'no-buttons)
                              (and (string= mime-type "text/plain") (<= nth 1)))
-                   (notmuch-show-insert-part-header nth mime-type content-type (plist-get part :filename))))
+                   (notmuch-show-insert-part-header nth mime-type content-type name)))
          (content-beg (point)))
 
     ;; Store the computed mime-type for later use (e.g. by attachment handlers).

But that doesn't work, because :content-description doesn't exist in
the part plist.  I've looked through the source for a bit and can't
figure out where that part plist is coming from.  Is it loaded from
notmuch output in notmuch-show-build-buffer?  I assume that
information comes from the index, in which case I'd need to tweak
_index_mime_part in lib/index.cc to add the description.  Indexing
descriptions seems like a generally useful thing, even outside of my
digest usecase (e.g. search image/jpeg attachements with “genome” in
their description [6]).  However, adding a field to the schema is more
invasive than changing the Emacs mode's attachment formatting; I
thought I should check in here for feedback and advice before wading
in with my —a̶x̶e̶— scalpel ;).

Thoughs?
Trevor

[1]: http://article.gmane.org/gmane.mail.rss2email/211
[2]: Digests: http://tools.ietf.org/html/rfc2046#section-5.1.5
[3]: Filename: http://tools.ietf.org/search/rfc2183#section-2.3
[4]: Filename hint to notmuch-show-insert-part-header:
     http://git.notmuchmail.org/git/notmuch/blob/HEAD:/emacs/notmuch-show.el#l883
[5]: Content-Desciption:
     http://tools.ietf.org/html/rfc2045#section-8
[6]: Content-Description examples:
     http://tools.ietf.org/html/rfc2183#section-3

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