nemo-file.c: Allow files with backend-provided previews to be thumbnailed, even if their filesystem forbids it. Nemo's thumbnailer now returns the provided preview icon if one exists. Actual thumbnail visibility is still controlled by the user like normal. This will not guarantee thumbnails, if the device doesn't have any, but the couple Android devices I tried did have them. ref: https://github.com/linuxmint/nemo/issues/3055 https://github.com/linuxmint/nemo/issues/3458 https://github.com/linuxmint/nemo/issues/3291 https://gitlab.gnome.org/GNOME/nautilus/-/issues/1921