code: public MediaType getContentType() {
          if (isNull(this.contentType)) {
              contentType = getHeader(HeaderName.CONTENT_TYPE)
                  .map(MediaType::of)
                  .orElse(WILDCARD);
          }
  
          return contentType;
      }
docs: Returns a content-type header value
  
  @return content-type header value in {@link Optional} object