Produce correct MIME type for SVG files

Could be easily extended to handle other MIME types, too.
@solidsnack hey there, thanks for the PR. I've not looked at this repo for quite a while :-) so I'm just double checking whether the `image/` is required or not? I would have imagined it to look like...

```rb
when '.svg'
  'svg+xml'
else
  extension[1, extension.length-1]
```