Where
Settings → Data and Storage → Automatic Media Download, configurable separately for mobile, Wi-Fi and roaming.
Recommended
- Mobile: photos only, no videos or large files;
- Wi-Fi: photos and videos with a size cap around 50 MB;
- Roaming: everything off.
Exceptions
- Enable auto-download for important chats in their settings;
- Anything not downloaded automatically can be fetched with one tap;
- Review and clear per-type usage under Data and Storage.