bug#36572: Proposal: mm-default-filename-template

["積丹尼 Dan Jacobson" <jidanni@jidanni.org>]

Don't you hate when you have to fill in the blank to an empty
Save MIME part to: ~/
prompt?

Sure one can set mm-default-directory, but what about the filename?

(Yes, one can cheat a little (setq mm-default-directory "/tmp/vvv-"))

So there should be some filename template variable, nil by default, that
will prompt to save to xyz-%d.%s e.g., xyz-001.html depending if that
file doesn't exist yet.

Even GIMP prompts with Untitled.jpg.
Even  ... prompts with ...

Let's take a look at a typical HTML mail,

! MIME-Version: 1.0
! Content-Type: text/html; charset=utf-8
! Content-Transfer-Encoding: base64

So here we can already know we can help the user put an .html at the end
of the proposed target filename.

Gnus v5.13
GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.4)
 of 2019-02-03, modified by Debian