Why iPhone photos are in HEIC
HEIC (High Efficiency Image Container) stores photos with the HEVC codec, so files are about half the size of a JPG at similar quality. As a result, many apps, registration forms and older Windows systems can’t open them yet. JPG is accepted almost everywhere, so converting is the most practical fix.
On an iPhone you can also choose Settings, Camera, Formats, Most Compatible so new photos are saved as JPG. This tool is useful for photos that are already in HEIC.