code: public static HashMap<String, ResourceType> getDefaultMapping() {
          HashMap<String, ResourceType> map = new HashMap<>();
          for (ResourceType rt : getResourceTypes()) {
              for (String extension : rt.extensions) {
                  map.put(extension, rt);
              }
          }
          return map;
      }
docs: Can be used as a base to customise mime types via {@link ResourceHandlerBuilder#withExtensionToResourceType(Map)}
  @return Returns the built-in mapping of file extension to resource type that can be added to in order to customise resource types.