Receive images in WhatsApp batches

Receive images in WhatsApp batches

WhatsApp stores supported images before it acknowledges their messages,
so a stop cannot lose them. A download has its own limit of two minutes,
apart from the ten seconds of the name lookups. Batch text keeps the
image markers and captions on their own lines, so the agent and a routed
child can read the pixels. Failed images and other visual media keep
short text markers instead of stopping the batch. View-once messages
stay unread.

A failed download logs only a short chat hash and a safe cause, such as
an HTTP status, because the library error text can hold signed media
URLs. Tests use made-up images and a fake download client to check
storage, the limits, the markers, routed delivery, and restarts.

Issue: 441b3c7