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Resolve one attachment on a message to a media url that works right now. Instagram and Facebook sign DM media urls per request and expire them, so the `url` on a message is a snapshot: it works when you read the message and stops working later. This endpoint checks the stored url and, when it has gone stale, re-mints the message's media from Meta and persists it before answering. The message id never expires, so this URL is the one to store. It is returned ready-made on each attachment as `refreshUrl` when you read a message over REST. **Webhook payloads do not carry `refreshUrl`**, so a webhook-driven integration builds this URL itself. Every piece is in the event: `message.conversationId`, `message.platformMessageId`, the attachment's zero-based position, and `account.accountId`. **`accountId` is a required query parameter**; omitting it returns `400` `missing_required_field`, which is the same requirement `GET /v1/whatsapp/media/{mediaId}` has. By default it responds `302` to the live media url, so it can be used directly as an `<img src>` on a browser session. API-key integrators should pass `?format=json` and read `url` off the body, since a browser cannot attach an Authorization header to an image request. Only Instagram and Facebook media can be re-minted. On other platforms the stored url is returned as-is when it still resolves, and `404` otherwise.