{
  "Version": "2008-10-17",
  "Statement": [{
    "Sid": "AllowPublicRead",
    "Effect": "Allow",
    "Principal": {
      "AWS": "*"
    },
    "Action": [ "s3:GetObject" ],
    "Resource": [ "arn:aws:s3:::MY_BUCKET_NAME/*" ]
  }]
}

https://schier.co/blog/2014/09/11/how-to-make-files-uploaded-to-s3-default-to-public.html