bug#43865: mml-attach-file could call system file chooser

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: 積丹尼 Dan Jacobson <jidanni@jidanni.org>,
>   43865@debbugs.gnu.org
> Date: Thu, 08 Oct 2020 14:28:16 +0200
> 
> True, but I think this was more of a feature request. How about we
> allow use-dialog-box and use-file-dialog to have a value of 'always?

Does that really work?  Doesn't the mouse click even bring some
information needed by the dialog?  I guess it's okay if not, although
I wouldn't be building such a new feature on a single request.  But
that's me.