Plugin resources not served when plugin installed in ${confluence.home}/plugins

I created a custom theme plugin with an included CSS file as a plugin resource. When I install the plugin through the administration interface into ${confluence.home}/plugins, it seems to work otherwise fine but the CSS file is not served by Confluence under .../download/resources/.../style.css. Everything works properly when I move the plugin jar from ${confluence.home}plugins into .../WEB-INF/lib.

I suspect that the /download/* servlet is not looking at the plugin directory when serving downloadable plugin resources.