Upload file data to an existing Attachment as the second step of the supported legacy two-request workflow. First POST JSON metadata to /attachment.json, including related_object, related_object_uuid, and file_type (for example, .pdf), then use the returned x-record-uuid here. Send the non-empty file bytes as an application/octet-stream request body, or send one file using multipart/form-data. For raw uploads the existing record supplies the file extension; for multipart uploads include the extension in the uploaded filename and ensure it matches the record's file type. Authenticate with an API key or an OAuth token with manage_attachments. Send Accept: application/json for the JSON success response; legacy clients may receive a text/html response. For new integrations, use the single multipart POST to /attachment.json described in the attachment guide.