Create Invoice Note or Attachment (Legacy)

Legacy combo endpoint that can add a note and/or an attachment to the invoice in one call, nested under a single invoice object (not array-based, unlike the modern attachments endpoints). If both notes and attachments are sent in the same call, only the notification for notes fires (the notification type defaults to attachment and is overwritten to note). Adding a note also bumps the invoice's association count; the modern attachment endpoints do not.