Upload a document to attach to an order — typically signed agreements, identity-verification PDFs, or other partner-supplied attachments that the state filing workflow needs.

Encoding: the request must be multipart/form-data with the file in the file field and the order ID in the orderId field. Sending JSON returns 400.

Note on orderId: the field is named orderId here (not orderGuid as elsewhere). Use the same Salesforce-prefixed identifier returned by POST /create-order.