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

[Robert Pluim]

>>>>> On Thu, 08 Oct 2020 14:58:52 +0300, Eli Zaretskii <eliz@gnu.org> said:

    >> From: 積丹尼 Dan Jacobson
    >> <jidanni@jidanni.org>
    >> Date: Thu, 08 Oct 2020 19:45:59 +0800
    >> 
    >> C-c C-a runs the command mml-attach-file (found in message-mode-map),
    >> which is an interactive autoloaded compiled Lisp function in
    >> ‘mml.el’.
    >> 
    >> OK, but wouldn't it be nice if this could call the "system file chooser"
    >> like Firefox and Chromium do when picking files?

    Eli> Emacs pops up the file selection dialog when the command that prompts
    Eli> for a file was invoked from a mouse-click event (some menu or tool-bar
    Eli> button).  When the command is invoked from the keyboard, we don't show
    Eli> the GUI dialogs.  See the variable use-dialog-box.

    Eli> This is not a bug, but the usual Emacs behavior.

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?
This might even be a good 'starter pack' setting.

Robert
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