HEIF stands for the standard: High Efficiency Image Format; while HEIC is Apple‘s chosen file name extension.
On Fedora 39 KDE Plasma spin, its Dolphin file manager and Gwenview image viewer don’t support HEIF/HEIC image files. You can solve the problem easily.
a) Installing
Install a package named libheif-freeworld.
sudo dnf install libheif-freeworld
b) Refreshing
Refresh/reload/restart your Dolphin file manager or Gwenview image viewer or the software you want to take effect.
Enjoy!