Nautilus doesn’t seem to display thumbnails for PDFs or videos. Presumably it’s because Evince and Totem are flatpaks. Does anyone know if there is meant to be support for generating thumbnails with flatpak’d apps?
As a workaround I layered evince-thumbnailer to get PDF thumbnails.
Wild guess, should flatpaks export .thumbnailer files similar to .desktop (reading this thread https://github.com/flatpak/flatpak/issues/1286#issuecomment-354554684)?