This 5 Seconds of Summer photo contains
fedora, felt hat, homburg, stetson, trilby, dress hat, high hat, opera hat, silk hat, stovepipe, top hat, topper, beaver, boater, leghorn, panama, panama hat, sailor, skimmer, and straw hat.
There might also be
porkpie, porkpie hat, pith hat, pith helmet, sun helmet, topee, topi, and campaign hat.