/usr/share/icons/oxygen/256x256/mimetypes/text-rtf.png is in oxygen-icon-theme-complete 4:4.8.2-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/icons/oxygen/256x256/mimetypes/application-vnd.oasis.opendocument.text-master.png