code: private void respondHeadersOnlyForFirstChunk() throws Exception {
          // doneResponding is set to true by respondLastChunk
          if (doneResponding) {
              throwDoubleResponseError();
          }
  
          if (nonChunkedResponseOrFirstChunkSent) {
              return;
          }
  
          if (!response.headers().contains(CONTENT_TYPE)) {
              response.headers().set(CONTENT_TYPE, "application/octet-stream");
          }
  
          // There should be no CONTENT_LENGTH header
          response.headers().remove(CONTENT_LENGTH);
  
          setNoClientCache();
  
          // Env2Response will respond only only headers
          respond();
      }
docs: If Content-Type header is not set, it is set to "application/octet-stream"