code: public static String getContentType(String ext) {
          String ct = extToContentType.get(ext);
          if (ct == null)
              ct = isBinary(ext) ? "application/octet-stream" : "text/plain";
          return ct;
      }
docs: If not mapped, content-type is application/octet-stream for binary and test/plain otherwise.
  TODO: extensibility