code: public ChannelFuture respondBinary(ByteBuf byteBuf) throws Exception {
          if (HttpUtil.isTransferEncodingChunked(response)) {
              respondHeadersOnlyForFirstChunk();
              return channel.writeAndFlush(new DefaultHttpContent(byteBuf));
          } else {
              if (!response.headers().contains(CONTENT_TYPE))
                  response.headers().set(CONTENT_TYPE, "application/octet-stream");
  
              response.headers().set(CONTENT_LENGTH, byteBuf.readableBytes());
              response.content().writeBytes(byteBuf);
              return respond();
          }
      }
docs: If Content-Type header is not set, it is set to "application/octet-stream".
  
  @param byteBuf Will be released