Export the supporting doc of the Order.

A file produced by a supplemental form has formSubmissionId and formFileRole. formSubmissionId matches the entry in Get form data and the fundsub.order.supporting_form.status.updated webhook. formFileRole is formDocument for the filled or signed form, or a file uploaded for the form, and signingCertificate for the e-signature certificate. Both fields are absent on files uploaded without a form. A file used by two form instances is listed once for each instance.