It is hard to use SugarMessageBody with introspection, e.g. from python.
The data itself is exposed in the struct:

typedef struct {
	const char *data;
	goffset     length;
} SoupMessageBody;

However there is currently no way to produce an annotation for this that says "data is a byte array with length length". Instead data is interpreted as a NULL-terminated string, which is wrong.

I believe the norm here would be to provide an API to get at the data. There is actually a way of doing that, so maybe this bug could be closed simply by mentioning it in the documentation: call soup_message_body_flatten() to convert the body into a SoupBuffer then use soup_buffer_get_as_bytes().