Download an attachment

Downloads exact retained inbound bytes using this account’s Bearer key. URLs are stable and require Authorization on every request. Pending capture returns 409 attachment_not_ready with Retry-After; failed or expired retained assets return 410 attachment_unavailable; missing, pruned, outbound or foreign assets return 404 attachment_not_found. Retention is 30 days from server receipt and downloads do not extend it. GET admitted before expiry may finish from acquired bytes. Range is ignored (full 200 response); HEAD is refused.