application/octet-stream as default content-type

If no content-type is found (because the extension is missing/not provided), we return `null`. I think it should be better to return `application/octet-stream` to force the browser to consider it as a binary and therefore download it. I am just following the recommendations of the [RFC2046](http://tools.ietf.org/html/rfc2046), where for all unrecognized files (images, videos, etc.), it is proposed:
>    Unrecognized subtypes of "[…]" should at a miniumum be treated as "application/octet-stream".

Thoughts?
@Hywan thanks much better ;)