curl --request GET \
  --url https://api.upwell.com/api/rest/invoices/{invoiceId}/documents

{
  "invoiceDocumentsByInvoiceId": [
    {
      "createdAt": "<unknown>",
      "documentId": "<string>",
      "filename": "<string>",
      "id": "<string>",
      "invoiceId": "<string>",
      "sourceSystem": "<string>",
      "sourceSystemId": "<string>",
      "type": "<string>",
      "updatedAt": "<unknown>",
      "url": "<string>"
    }
  ]
}