mimetype specific cache control not honoured for external images

[abovedave]

In the config file I've set specific cache expiration times for the image/jpeg mimetype.

"headers": {
    "useGzipCompression": true,
    "cacheControl": {
      "default": "public, max-age=3600",
      "paths": [],
      "mimetypes": [
        { "image/jpeg": "public, max-age=86400" },
        { "text/css": "public, max-age=86400" },
        { "text/javascript": "public, max-age=86400" },
        { "application/javascript": "public, max-age=86400" }
      ]
    }
  }

Url:
http://localhost:8001/http://www.images.co.uk/images/images/image_data/resizeandpad:700:700/58714?w=300&ratio=2-3&resize=aspectfill&gravity=center&q=30

However the external file return by CDN uses the default value: