def guess_content_type_from_body(body):
    """ Guess the content-type based of the body.
    
    * "text/html" for str bodies starting with ``<!DOCTYPE html>`` or ``<html>``.
    * "text/plain" for other str bodies.
    * "application/json" for dict bodies.
    * "application/octet-stream" otherwise.
    """
    if isinstance(body, str):
        if body.startswith(('<!DOCTYPE html>', '<!doctype html>', '<html>')):
            return 'text/html'
        else:
            return 'text/plain'
    elif isinstance(body, dict):
        return 'application/json'
    else:
        return 'application/octet-stream'
