So a multipart form "zip bomb"? Sounds like the kind of thing that would do... well, that. But also, by that point you don't even need a WAF to block it, IIRC multipart forms can be disabled or their memory limit clamped on php.ini.
(Not that I'd rely on that; not everyone has access to php.ini. But then again, we are talking about a world where there are more than one (1) ways of dealing with problems)