Multiple subrequests, each with a proxy_pass

Server NGINX
1

For a certain endpoint (/upload), I would like to sometimes (when
content-type matches) create a subrequest which calls an upstream with
proxy_pass to include content before the actual response, which is also
provided via proxy_pass. This isn’t working right now and is only
returning the content of the subrequest’s upstream. Is this even
possible? Here’s the config that I’m using:

[code]
location "/" {
    if ($content_type ~ "^multipart/form-data") {
        echo_subrequest $request_method /saver2;
    }

    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header Host $http_host;
    proxy_pass http://assets;

}

location /saver2 {
    internal;
    rewrite / /__testing__/saver break;
    proxy_pass http://filesaver;
}

[/code]

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