I think this might seem a little odd a first, but all former "engines" are going to need a mime type even if a made up one. One thing this enables is the ability to request the file as the original source. `find_asset("foo.js", accepts: "text/coffeescript")` would give you the original `foo.coffee`.

The old `register_engine` will be a short hand for the following

``` ruby
register_engine ".foo", FooTemplate

# use an internal sprocket mime prefix
register_mime_type "sprockets/foo", ".foo"
register_transformer "sprockets/foo", "application/javascript", FooTemplate
```