Code from `setUpFilePreview` as suggested in review; however, this will fail if `mime` is upper case. FWIW, [RFC 2045](http://tools.ietf.org/html/rfc2045) says:

 > The type, subtype, and parameter names are not case sensitive.

That may not be an issue for us if we're always generating the mime property ourselves (e.g. not relying on an oembed provider).