I just realized that iOS 11 handles screenshots taken on device in a very interesting way. If you immediately send screenshot after it was taken - JPEG file will be send. If you take screenshot and save it to Photos, and then share it from Photos - PNG file will be send.
Is there reasonable explanation for it? Settings I'm missing? I'd prefer to always have screenshots in PNG.
Tested on iPhone X / iOS 11.2.6 and iPad Pro / iOS 11.0