"attachment": {
  "content_type": "image/jpeg",
  "byte_size": 1597,                // size in bytes
  "url": <download_url_string>,     // download link
  "filename":"test.jpg",
  "signed_id": "AWL32fnufhOFHPWIFHsbfkljbf43kbg2wv-KJfdf32423Kbfwkbgfwe4sdf6sdfjsdfs"
}