Plugin not getting called

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1

I am trying to build a converter plugin. I have started from the template provided in the docs:

# _plugins/upcase_converter.rb
module Jekyll
    class UpcaseConverter < Converter
        safe true
        priority :low
        
        def matches(ext)
            ext =~ /^\.upcase$/i
        end
        
        def output_ext(ext)
            ".html"
        end
        
        def convert(content)
            puts "===== #{content}"
            content.upcase
        end
    end
end

I can see the plugin in my jekyll serve log immediately, but documents with the .upcase extension just get copied to the _site directory as are with no changes and my test puts line doesn’t get called. How do I troubleshoot this?