PetaPixel:

After updating your iPhone to iOS 11, you may have noticed that your photos are all .heic files instead of the ubiquitous .jpeg. If you’d rather go back to shooting JPEG for now until you feel more ready to make the switch, it’s actually extremely easy to do.

The new .heic format seems great but without widespread adoption, many users might want to switch back to the more common/compatible JPEG format.