Complete an attachment upload sessionBeta

Finalize an attachment upload session after the file has been uploaded to the pre-signed URL. Nylas verifies that the upload succeeded in storage and, if size was declared on session creation, that the uploaded byte count matches the declared size.

After completion, reference the attachment in a send or draft request by passing { "id": "<attachment_id>" } in the attachments array.

Retention note: By default, the uploaded file is deleted from storage when expires_at passes (one hour after session creation). Plan your send to occur inside this window. The attachment metadata row is retained for 60 days for observability.

For the full upload flow, see Send large attachments.