Why HEIC files cause so much trouble
HEIC is genuinely excellent at compression — it stores roughly half the bytes of an equivalent JPEG for the same visible quality. The trade-off is compatibility. HEIC is a container format based on HEIF, and decoding it requires the High Efficiency Video Codec. Safari on macOS and iOS can do this natively. Google Chrome, Firefox, Windows Photos, Adobe Photoshop (before v24), and essentially every web upload form cannot. That leaves you with photos you cannot open or share. Converting to JPG produces a file that has been readable on every device since 1992, which is why it remains the default format for sharing, printing and archiving photos.