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

[積丹尼 Dan Jacobson]

>>>>> "EZ" == Eli Zaretskii <eliz@gnu.org> writes:
>> See the variable use-dialog-box.
EZ> And the companion variable use-file-dialog.

Testing with emacs -q:
Both are "'t", but C-c C-a still doesn't turn on the file chooser.
I still need to click "Attach File" from the tool bar to do that.
But I want C-c C-a also to do that. But it doesn't.
emacs-version "26.3",

We also note some side effects mentioned
"On some platforms, file selection dialogs are also enabled if this is
non-nil."

I hope all this isn't too bundled together.

(I want a way to have C-c C-a call the file chooser,
but have both variables still nil (which is in fact how I usually have them.)