Adding first attachment or removing last attachment to an invoice should trigger the invoice webhook
Currently if you add an attachment to the invoice where none existed before, the invoice webhook is not fired, even though invoice.hasAttachments is changed to be true.
Same for if you remove the last attachment on an invoice, this should trigger the invoice webhook as the hasAttachments is now false.
Currently to get an invoice attachment you need to make a call to the attachment endpiont for a specific entity id, then you need to make a call to the download end point for each found attachment.
We need an efficient way to be notified of weather there is an attachment or not, othewise we will quickly hit our api quote just looking up attachments on invoices.