(2009, to be exact). It wouldn't have been nominated if there were only 5 nominees. Also, this year there were slim pickings for Best Picture, compared to most other years — that helps too.
BTW, the Dark Knight and Batman Begins were released before the number of nominees expanded, IIRC. Had they been released afterward, I think they both might have been nominated.
OTOH, Interstellar didn't get nominated, even after expansion. So sometimes the Academy just has bad taste.