downloading image failed [json-pointer:\/blocks\/0\/image_url]

[otiai10]

% curl -XPOST "https://slack.com/api/chat.postMessage" \
-H "Content-Type: application/json" \
-H "Authorization: Bearer $TOKEN" \
-d '{
        "channel": "otiai10-dev",
        "blocks": [
                {
                        "type": "image",
                        "image_url": "http://localhost:8080/foo/baa",
                        "alt_text":"test"
                }
        ]
}'

{
  "ok":false,
  "error":"invalid_blocks",
  "errors": [
    "downloading image failed [json-pointer:\/blocks\/0\/image_url]"
  ],
  "warning": "missing_charset",
  "response_metadata": {
    "messages": [
      "[ERROR] downloading image failed [json-pointer:\/blocks\/0\/image_url]"
    ],
    "warnings": [
      "missing_charset"
    ]
  }
}