They haven't created the file chooser portals yet. But I saw a comment from mmstick a few months ago on reddit that he said it will support it. I mean it's a simple basic- feature, gnome could've had it if they cared.
We have been developing our own toolkit, libcosmic, which is being used for everything. From the lock screen, compositor, applets for the desktop, and the applications themselves. There is an examples directory for third party developers to learn hope to build their own applets and applications with.