Download an attachment

Streams the raw attachment bytes with the original Content-Type and Content-Disposition headers.

A PNG, JPEG, GIF or WebP is read for hidden instructions before an API key or agent is given it, because a model can look at an image the way it reads a body. If the text in it reads like instructions aimed at an AI, or it cannot be checked, this returns 403 IMAGE_HELD and a person can still open it in the dashboard. 503 IMAGE_HELD means it could not be checked right now and is worth retrying. A signed-in person in the dashboard is never held, and neither is an inbox or account that has turned mail protection off.