Comment # 2 on bug 1136753 from Dario Tislar

Well, I wasn't sure if it's Xfce only, but xdg-desktop-portal is not enough,
you also need xdg-desktop-portal-gtk. There is also xdg-desktop-portal-kde in
repos, but I didn't need that one (maybe that one is only for plasma?). 
So based on above I thought this is Xfce issue, but I can change it to whatever
you want.