Why does mapMulti need type information in comparison to flatMap

Question

I want to use mapMulti instead of flatMap and refactored the following code:

// using flatMap (version 1) => returns Set<Item>
var items = users.stream()
                 .flatMap(u -> u.getItems().stream())
                .collect(Collectors.toSet());

into this (version 2):

// using mapMulti (version 2) => returns Set<Item>
var items = users.stream()
                 .<Item>mapMulti((u, consumer) -> u.getItems().forEach(consumer))
                 .collect(Collectors.toSet());

Both return the same elements. However, I am in doubt if I should really replace all my flatMap with that more verbose code of mapMulti. Why do I need to add the type information before mapMuli (.<Item>mapMulti). If I don't inlcude the type information, it will return a Set<Object>. (How) can I simplify mapMulti?