docs: public CmsMimeType addMimeType(String extension, String type) throws CmsConfigurationException {
  
          // check if new mime types can still be added
          if (m_frozen) {
              throw new CmsConfigurationException(Messages.get().container(Messages.ERR_NO_CONFIG_AFTER_STARTUP_0));
          }
  
          CmsMimeType mimeType = new CmsMimeType(extension, type);
          m_configuredMimeTypes.add(mimeType);
          return mimeType;
      }