/usr/lib/rhythmbox/plugins/alternative-toolbar/ is in rhythmbox-plugin-alternative-toolbar 0.18.0-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/rhythmbox/plugins/alternative-toolbar/alternative-toolbar.plugin
/usr/lib/rhythmbox/plugins/alternative-toolbar/alternative-toolbar.py
/usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_controller.py
/usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_plugins.py
/usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_preferences.py
/usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_rb3compat.py
/usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_repeat.py
/usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_sidebar.py
/usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py
/usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_widget.py