EmailDeliverAttachment

One attachment: its name and the file, base64 encoded.

{
    "attachmentName": "invoice-2026-04.pdf",
    "content": "JVBERi0xLjQKJcfsj6IKNSAwIG9iago8PC9MZW5ndGggNiAwIFI+PgpzdHJlYW0K",
    "attachmentId": "string"
}