If s3fs cannot read file, it is possible that ACL has issue. One method is make object public.
We use ncml-lambda as bucket name. The following is Python lambda code.
s3 = boto3.resource('s3')
s3.meta.client.upload_file(tmpname, 'ncml-lambda', oname)
object_acl = s3.ObjectAcl('ncml-lambda', oname)
result = object_acl.put(ACL='public-read')