extensions option not working

I have been trying to figure out how to make this plugin only run on `.css` files, and exclude `.scss` files. I thought I could accomplish this by setting `extensions: ['.css']`, but this still runs on the `.scss` files. How can I exclude files?

The reason I want to run rollup-plugin-sass on the `scss` and rollup-plugin-postcss on the `css` is because I want to inject some third party css into the head, but I want to create separate css theme files from my scss as well. If that is possible here, I could just use one plugin, but I cannot figure it out.