Uploads a document

Attaches a document (PDF, TXT, JPG/JPEG, PNG, GIF, XLSX/XLS, DOCX/DOC, CSV, RTF, TIFF/TIF, BMP, Pages, Numbers, PPTX, or PPT) such as an invoice, contract, or receipt to an existing bill using multipart form data with a single file part. Call after the bill exists so approvers and auditors can open the evidence directly from the payable record. PDF, JPEG, and PNG keep content-signature validation and do not require a matching filename extension. Every other format requires an allowed filename extension and either a matching content type or application/octet-stream. Files detected as HTML are rejected regardless of their filename or declared content type.