Default contentType for Resources whose content we can't guess

URLConnection.guessContentType doesn't know about, e.g. MS Office types. Until we're on JDK 7 and can use Files.probeContentType (http://docs.oracle.com/javase/7/docs/api/java/nio/file/Files.html#probeContentType(java.nio.file.Path)), we use "application/octet-stream" as a fall-back.