Wondering if you are backing up your iPhone and iPad before deleting anything, so you can always retrieve files.
See Apple Support's online article for how to view the "recommendations" ... includes deleting large attachments:
How to check the storage on your iPhone and iPad - Apple Support
Else if you don't see the recoommendations listed there:
Look In the Storage section of Settings:
where your device might offer recommendations for optimizing your storage.
To optimize your storage:
- Tap "Show All" to see the recommendations (including delete large attachments) for your device.
- Read the description of each recommendation, then tap Enable to turn it on
or tap the recommendation to review the contents you can delete.