How to have thumbnails in file dialogs in certain GTK applications
You can have large thumbnails in the file picker in certain GTK 3 applications. Only a minority of applications supports this feature, but this minority includes Firefox and Thunderbird. The trick, surprisingly, is to force GTK applications to use KDE dialogs. You don’t need to use KDE itself, and the instructions assume you don’t.
The following instructions have been tested in Ubuntu 20.04. It should be possible to use them in other Linux distributions with small adjustments.
Install the required packages
sudo apt install kio-extras xdg-desktop-portal-kde
Enable the KDE file chooser
sudo sed -i "s/\(UseIn=.*\)/\1;$XDG_CURRENT_DESKTOP/" /usr/share/xdg-desktop-portal/portals/kde.portal echo GTK_USE_PORTAL=1 >> ~/.pam_environment
Log out and log back in
- Replace GTK+ file dialog with alternative. Super User.
- Dolphin not showing preview of images or any files. Ask Ubuntu.
GtkFileChooserNative. GNOME Developer Center.