curl --request POST \
  --url https://api.e-invoice.be/api/documents/{document_id}/attachments \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: multipart/form-data' \
  --form file='@example-file'

{
  "file_name": "<string>",
  "id": "<string>",
  "file_type": "application/pdf",
  "file_size": 0,
  "file_url": "<string>"
}