post https://api.apicbase.com/api/v2/procurement/purchase_orders//invoices/
Attaches an invoice file to a purchase order.
A purchase order can have up to 10 invoice files attached. One file can be uploaded per request. PDF and image formats are supported. The file size limit is 20 MB. Invoice files can be attached to purchase orders after they have transitioned to the SENT state. It is also possible to attach invoice files to delivered or partially delivered orders.