Attaches a receipt (PDF, TXT, JPG/JPEG, PNG, GIF, XLSX/XLS, DOCX/DOC, CSV, RTF, TIFF/TIF, BMP, Pages, Numbers, PPTX, or PPT) to an existing reimbursement using multipart form data with a single file part.
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.