The extension is rhtml because it uses a templating language called Rubinius that is based on Ruby.
Sproutcore documentation
Matthijs Langenberg's daily thoughts
About Matthijs Langenberg
I'm a 22 year old software developer from The Netherlands. I ♥ working with Ruby and Rails, doing iPhone development, using git … whatever makes me feel like pro. Currently working for a great company called Nedap. Feel free to contact me at mlangenberg@gmail.com or give me a tweet @mlangenberg.