code: public ContentTypeEngine registerContentTypeEngine(Class<? extends ContentTypeEngine> engineClass) {
          ContentTypeEngine engine;
          try {
              engine = engineClass.newInstance();
          } catch (Exception e) {
              throw new PippoRuntimeException(e, "Failed to instantiate '{}'", engineClass.getName());
          }
          if (!engines.containsKey(engine.getContentType())) {
              setContentTypeEngine(engine);
              return engine;
          } else {
              log.debug("'{}' content engine already registered, ignoring '{}'", engine.getContentType(),
                  engineClass.getName());
              return null;
          }
      }
docs: Registers a content type engine if no other engine has been registered
  for the content type.
  
  @param engineClass
  @return the engine instance, if it is registered