Uploads a document to an invoice

Attaches one PDF, JPEG, PNG, or XLSX file to an existing invoice. Existing attachments are preserved, including files uploaded through the Rillet application. Repeating an upload creates a new file; there is no idempotency key. When Update approval is required, returns 202 with a review reference and the file is not attached until approval. A different upload while an edit is pending returns 409. Cancel or resolve a pending approval before uploading another file. An invoice can have at most 10 attachments totalling at most 7 MiB, including files uploaded through the Rillet application, so the invoice email stays within delivery limits. A file name may have at most 255 characters and must not contain a path. Attachments are included when Rillet emails the invoice. Attach before the invoice is sent: adding a file to an invoice that was already sent does not send it again.