Also make sure to generate previews for .can files.
OS X makes some decisions about how thumbnails are displayed using the file UTI. By stating that these formats conform to `public.plain-text` instead of just `public.text`, OS X will display the file extension as a 'badge' over the thumbnail. UTI declarations within installed apps will override any declarations within Quick Look generators, so if users have apps with conflicting information, the badge may not display.
If a CFURLRef is created and then cast to an NSURL, the CFURLRef needs to be released. This is done through a `__bridge_transfer` which tells ARC to release it for us.