if($request_uri = "/filename.xml") {
    rewrite .* /bucketname/bucket-sub-name$request_uri break;
    proxy_pass https://s3-ap-southeast-2.amazonaws.com;
}

if($request_uri = "/filename.xml"){
    proxy_pass https://dev.example.com$request_uri;
}

if($request_uri = "/filename.xml"){
    proxy_pass https://s3-ap-southeast-2.amazonaws.com/bucketname/bucket-sub-name$request_uri;
}