Sinatra doesn’t know this ditty.

Try this:
# in main.rb
class GrapesRV
  get '/_detail/info:%D0%B1%D0%B5%D0%B7%D1%8B%D0%BC%D1%8F%D0%BD%D0%BD%D1%8B%D0%B9.jpg' do
    "Hello World"
  end
end