Redirects to a short-lived URL for the media on a received WhatsApp message. Inbound media is stored because WhatsApp's own URL is not fetchable without our credentials; this endpoint is what the `url` on the message's `image`, `video`, `audio`, `sticker` or `document` points at. The response is a `302` whose `Location` is a pre-authorized storage URL valid for 15 minutes, so your client must follow redirects and must not send its `Authorization` header to the redirect target. Media is kept for 30 days after the message is received; the message itself is kept longer, so a message older than that still lists its media's `mime_type` and `caption`, and this endpoint returns `410` once the bytes have expired. Outbound messages have no media to serve.