What I've done is installing
xdg-desktop-portal
libsForQt5.xdg-desktop-portal-kde
libsForQt5.kdialog
and adding
xdg.portal.enable = true;
xdg.portal.extraPortals = [ pkgs.libsForQt5.xdg-desktop-portal-kde ];
environment.sessionVariables.XDG_CURRENT_DESKTOP = "kde";
to my configuration.nix, but while I do get the qt file picker on my web browser, the theming is broken and it looks like this
I currently handle my theming through qt5ct with the line
environment.sessionVariables.QT_QPA_PLATFORMTHEME = "qt5ct"