It is a little odd to me that I find OpenJDK JarURLConnection does not support bmp file. The testcase below:

public static void main(String[] args) throws MalformedURLException, IOException {
        JarURLConnection conn = (JarURLConnection) new URL("jar:file:/somedir/somejar.jar!/somebmp.bmp").openConnection();
        System.out.printf("Returned type for the entry should be the known type " +
                "image/bmp, but it is " + conn.getContentType());
    }

    (Need create a somejar.jar containing a bmp file named somebmp.bmp)

    It should return image/bmp but it does not. I see we may add image/bmp support in the windows\lib\content-types.properties to make the testcase pass. However I am not sure if OpenJDK do want to support it, or there is some reason why OpenJDK don't?

see discussion on net-dev mailing list:
  http://mail.openjdk.java.net/pipermail/net-dev/2011-March/002780.html