What is the text encoding of the MySQL Slow Query Log?

The documentation does not seem to contain the word "encoding", so I am looking for a reference on the encoding of the MySQL slow query log. Based on the slow query logs I have seen so far, my current guess is that it is ASCII rather than UTF-8, but there are also strange characters that look like raw bytes for binary columns, so I am wondering if anyone knows what the intended encoding is, and whether it is documented anywhere.