Sinatra doesn’t know this ditty.

Try this:
# in main.rb
class GrapesRV
  get '/_detail/info:tab:%D0%B7%D0%B0%D1%85%D0%B2%D0%B0%D1%82_%D0%BA%D1%80%D0%B5%D0%BF%D0%BE%D1%81%D1%82%D0%B5%D0%B9_3.jpg' do
    "Hello World"
  end
end