ALLOWED_IMAGE_TYPES, MAX_IMAGE_BYTES, and unique_image_name() were
duplicated identically in both the edit and serve servers. Centralising
them means a single change point for any future extension (e.g. adding
image/avif support).
Tests added in tests/test_shared_images.py cover no-collision, single
and chained collisions, no-suffix filenames, and constant values.
Davide Scaini