Accept.best_match currently checks for a "" in the offers and raises an exception. However, it's quite reasonable for an application to know how to return any representation of a resource, e.g. a PNG, GIF, etc version of an image. If the application was trying to decide where to route a request it would need to pass image/ to best_match as one of the offers.
You caught us doing some quick maintenance, temporarily restricting your repository to read-only access.
Issue #8 wontfix