There is no one - specific - item that is revealed to be inside the suitcase. This ambiguity is a key part of the movie's allure. It allows different viewers to have different ideas about what might be in there, whether it's a life - changing amount of cash or a dangerous and illegal item.
The filmmakers deliberately left it unclear what was inside the suitcase. This creates a sense of mystery and intrigue throughout the movie. It could be jewels, or important documents, or something completely different.