Reassigning to Gnome as it’s more about Gnome environment than desktop-base that’s about theming for Debian.
The program you must be refering to must be « assogiate » which was a GTK2 file association editor. It’s been deprecated and not packaged anymore starting with Jessie. Maybe the files association functionality available in Nautilus default file manager (right click, Properties, Open With tab) could be considered a sufficient replacement ? Regards, --Aurélien