Upload Attachment

Upload an image or text document to attach to a DM.

Returns {attachment_id, mime_type, size_bytes, width, height, thumb_url, full_url}. The attachment is created with message_id IS NULL and gets wired into a message later via the send endpoint (attachment_ids: [...]). Orphaned uploads (no message after 24h) are garbage-collected.

Validation: image/{jpeg,png,webp,gif} ≤ 10 MB, or text/{markdown,x-markdown,plain} ≤ 1 MB. EXIF is stripped on the server side so a sender doesn't inadvertently leak GPS coordinates. Text is checked as strict UTF-8 (the equivalent of the image decode) and served back as text/plain with Content-Disposition: attachment so it can never execute in our origin.

A document has no thumbnail: thumb_url is null and width / height are null.

Rate limit: 60 uploads per hour per user (lines up with the 60/h send limit — one attachment per message average).