Negotiate content for implied extensions using accept header


Transparent Content Negotiation  ([RFC2295](http://tools.ietf.org/html/rfc2295)) for requesting resources without extension.

Using it, the user-agent can indicate if it prefers a particular mime-type over the others. This is a nice way to transition to new file formats. It is still possible to serve the old format while using the same url.

```
/image.png
/image.webp
```

For the above image files, a request to `/image` results in most browsers receiving the png file. The chrome browser on the other hand will receive the webp variant as it sends the appropriate accept header.

I also took the liberty of adding some extensions to the allowed urls. Sorry.