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

[Robert Pluim]

>>>>> On Sat, 10 Oct 2020 22:17:05 +0200, Lars Ingebrigtsen <larsi@gnus.org> 
>>>>> said:

    Lars> Kévin Le Gouguec <kevin.legouguec@gmail.com> writes:
    >> I agree with Robert's interpretation of Dan's report here, this makes
    >> sense to me as a feature request.  Other Software™ usually present a
    >> visual file chooser whether the user clicked File > Open or typed
    >> Control+O, so some users could prefer the hypothetical 'always behaviour
    >> Robert suggested.

    Lars> I'm not opposed to a user option like that, but I do wonder whether 
it'd
    Lars> be used a lot: If `C-x C-f' popped up a graphical file chooser, I 
think
    Lars> I'd grow pretty tired of it.

You would, I would, but our hypothetical new user would be expecting
it.

    Lars> Perhaps I closed this bug report prematurely -- Dan has a point here:
    Lars> `C-c C-a' in Message is kinda special in one way, in that it's 
commonly
    Lars> used to insert images and the like into an email.  For images, you 
want
    Lars> the graphical file chooser, because it often shows you thumbnails and
    Lars> stuff.

    Lars> With `C-x C-f', the overwhelming number of things you'd open would be
    Lars> text files, and no thumbnails help with that.

This suggests we should have several options for this:

- never
- when invoked by mouse
- always
- only for specific commands

Robert
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