For detecting and responding to mobile user agents, I'm using

Mime::Type.register_alias "text/html", :mobile

and the approach I'm wondering what is the best approach to test with capybara. This article suggests setting up an iphone driver with Capybara.register_driver :iphone do |app|

http://blog.plataformatec.com.br/2011/03/configuring-user-agents-with-capybara-selenium-webdriver/

but I'd like a more flexible approach where the mime type is set via the url extension

localhost/index.mobile

and where I can do this

visit user_path( format: :mobile)

Rails understands the extension and sets the format in the params hash, but how do I get the url helper methods to add that to all urls as a file extension?