Get a message attachment

Downloads the attachment at the zero-based index in a message's content.attachments array and returns its decrypted bytes with the media type the attachment declares. A declared PNG, JPEG, GIF or WebP type is kept only when the bytes match it; a missing, malformed or mismatched type returns application/octet-stream. The Content-Disposition header carries the original filename. Works for attachments a customer sent and for attachments you sent. For a received attachment, message_id is the message.id of the amb.received webhook event, and index is the attachment's position in that message's content.attachments array.

Apple retains an attachment for 30 days, renewed each time its reference is reused; availability after that period is not guaranteed, so download an attachment when the message arrives rather than on demand later. An unknown message or index returns 404; an unavailable or expired Apple reference returns 503.