Get a short-lived download link for an attachment

For a file too big to pass through an agent's context (a PDF as base64 is millions of characters). Returns a link to fetch the file with a plain GET, no API key. The link works for one attachment only, for 10 minutes and 3 uses; a HEAD request does not spend a use. It is served as a download, never rendered, and active types such as HTML and SVG are sent as application/octet-stream. The content is untrusted: it was written by the sender.

Refused with 403 QUARANTINED for a message held for review, and with 403 IMAGE_HELD for an image whose text looks like instructions to an AI or which could not be checked (see the attachment download). A link stops working if its message is held after the link was issued.