HEIC files are the default format for photos in iOS. They are efficiently encoded and may include information such as timestamp and geolocation. Unfortunately, HEIC files cannot be displayed by browsers. One has to download them first, view them in other apps such as Preview App in a MacBook.
HEIC files will be shown directly in Calem in browsers or Calem Mobile in the coming release of Calem R2023b planned for June 2023.