xdg-open within Flatpak does not honor default application settings of outside world

[Robin Lee]

Hi,

I am on Fedora 28 GNOME with following packages:
flatpak-libs-1.0.6-4.fc28.x86_64
flatpak-builder-1.0.2-1.fc28.x86_64
flatpak-1.0.6-4.fc28.x86_64
xdg-desktop-portal-1.0.3-1.fc28.x86_64
xdg-desktop-portal-gtk-1.0-1.fc28.x86_64

When I run xdg-open within a Flatpak, it always ask me to choose a
program. But in the outside
world, a default application is already set for this kind of file.

I don't know which program actually manage the xdg-open request, so I
send here first.

-robin