Invoice API Attach PDF

[sg1valsag]

I try to attach a PDF to an invoice created with the API.   It appears to work, however, the attachment isn't on the invoice.  Below is the JSON I use to  add it to the invoice during invoice creation.   The URL use to create the invoice  does work.

"attachments": [ { "id": "Conditions", "reference_url": "https://url/filename.pdf", "content_type": "application/pdf" } ]

Thanks!