http://3wymlmcsvxiaqzmbepsdawqpk6o2qsk65jhms72qqjulk5u4bgmvs3qd.onion/fuss/arduino
In other words, from the point of view of the protocol, the payload can be a string, a file, and image, etc. By contrast, the topic parameter is indeed expected to be a string, hence the buffer pointer char *topic among the function parameters.