An iPhone shoots HEIC. The server makes a HEIC photo's thumbnail and preview with a decoder the computer provides. It is looked for at every start, and the log says which one it found.
macOS
Nothing to install.
Debian, Ubuntu and the Spark
sudo apt install libheif-examples libheif-plugin-libde265
On Debian 12 and Ubuntu 22.04 the decoder is built into libheif and the plugin package doesn't exist: install libheif-examples alone.
Ubuntu 24.04's libheif (1.17.6) fails on some HEIC files from iOS 18 and later; libheif 1.18.2 reads them. On 24.04, take libheif's authors' builds:
sudo add-apt-repository ppa:strukturag/libheif
sudo add-apt-repository ppa:strukturag/libde265
sudo apt update && sudo apt upgrade
Fedora
Fedora's own libheif has no HEVC decoder; RPM Fusion's adds it:
sudo dnf install https://mirrors.rpmfusion.org/free/fedora/rpmfusion-free-release-$(rpm -E %fedora).noarch.rpm
sudo dnf install libheif-tools libheif-freeworld
Then restart
systemctl --user restart diune-media-server
HEIC photos backed up before get their previews in the background, one at a time, so a backup under way isn't slowed.
Without a decoder
Everything else works. HEIC photos are still backed up and downloadable; they show no picture until a decoder is installed and the service restarted.