Fix run-file-chooser under flatpak

Since I added file filter support to xdg-desktop-portal, it's now fatal to select a current file filter that is not in the list of file filters. If we mess this up, there will be no file chooser dialog at all. Maybe the portal is being a bit too strict, but Epiphany is definitely in the wrong here for not using the file chooser interface properly.

We just need to add the filter to the list before showing the dialog.

Fixes: https://www.reddit.com/r/gnome/comments/baiqan/who_is_using_epiphany_how_is_your_experience/