emacs: Prefer Content-Description over filename for part buttons

emacs: Prefer Content-Description over filename for part buttons

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.  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 (which I just taught
notmuch-show to export), falling back on the filename parameter if
Content-Description isn't set.

[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