curl --request POST \
  --url https://app.sendmux.ai/api/v1/mailbox/attachments:upload \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/octet-stream' \
  --data '"<string>"'

{
  "ok": true,
  "meta": {
    "request_id": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx"
  },
  "data": {
    "blob_id": "<string>",
    "filename": "invoice.pdf",
    "content_type": "application/pdf",
    "size_bytes": 12345
  }
}