Comment 15 for bug 1171650

If someone is interested by this bug, here is how I think this should be done until the FileChooserDialog got more abstracted upstream.
I managed to build a Gtk3 module that detects every Gtk.FileChooserDialog opening, so the remaining work is to implement the same UI as Pantheon Files.
I think that this module should be a part of files allowing people to have this FileChooserDialog if they use Pantheon Files.
( https://code.launchpad.net/~tintou/pantheon-files/gtk-module )
for testing, just append "GTK_MODULES=pantheon-files-module" before running an app.
for example GTK_MODULES=pantheon-files-module noise

Then making Pantheon Files as a default app can add the module name in the org.gnome.settings-daemon.plugins.xsettings.enabled-gtk-modules DConf entry.