Auth key for attachment upload
An optional fields mask
INVOICE_FOR_BUYER, INVOICE_SUPPORTING, PURCHASE_ORDER, SALES_ORDER, DELIVERY_DOCUMENT, PAYMENT_PROOF, DISPUTE, DISPUTE_RESOLUTION, ENQUIRY, ENQUIRY_RESOLUTION, TAX_DOCUMENT, COMMERCIAL_AGREEMENT, COMMERCIAL_AGREEMENT_SUPPORTING, LEGAL, GENERAL, CREDITNOTE_FOR_BUYER, CREDITNOTE_SUPPORTING, TICKET, USAGE_ESTIMATE, MAIL_ATTACHMENT