use-file-dialog

use-file-dialog is a variable defined in `C source code'.
Its value is

t

Documentation:
Non-nil means mouse commands use a file dialog to ask for files.
This applies to commands from menus and tool bar buttons even when
they are initiated from the keyboard. If `use-dialog-box' is nil,
that disables the use of a file dialog, regardless of the value of
this variable.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.