Nginx API proxy form-data and custom header not passed to remote API

Question

I want to setup an Nginx with this purpose.

Context

• Serve Angular 2 dist static files from root url: http://example.com -> display the index.html at /home/www/index.html

• In my Angular2 project, I make some calls locally to /api/some-endpoints both in GET, POST.

• I want the calls to /api/{ANY-PATH} be redirected as is to a remote private url. I need the call be made with my custom headers (for example form-data or apikey header) AND add another apikey to the request to distant server.

My problem

When making a POST call (from machine where nginx is installed) to /api/document, with form-data values (containing an uploaded file) and my custom header: apikey , 253018b8425f4eb08291a1b68c4bc328

The distant server doesn't seem to receive nor the form-data nor the apikey.

My config

upstream api-server {
    server myprivate-api-server.com:8000;
}

map $http_apikey $api_route {
    default "error";
    253018b8425f4eb08291a1b68c4bc328 'api-server';
}

server {
        listen 80;
        index index.html;
        server_name example.com;

        location = /501_apikey.html {
                root /home/www/error_pages;
                internal;
        }

        location ~ /api/(?<path>.*) {

            if ($request_method = OPTIONS ) {           
                add_header 'Access-Control-Allow-Origin' "$http_origin";
                add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
                add_header 'Access-Control-Allow-Headers' 'apikey';
                return 200;
            }

            if ($api_route = "error"){return 501;}

            set $clientapikey "hW4g5V60UT2O3iQP1PS2g29hKjQ403E5";
            error_page 501 /501_apikey.html;

            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS, PUT';
            add_header 'Access-Control-Allow-Headers' 'apikey';
            add_header 'clientapikey' $clientapikey;

            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-NginX-Proxy true;    
            proxy_pass http://$api_route/$path$is_args$args;
            proxy_set_header X-Forwarded-For $remote_addr;
            proxy_set_header X-Forwarded-Host $remote_addr;
            proxy_ssl_session_reuse off;
            proxy_set_header Host $http_host;
            add_header 'Access-Control-Allow-Headers' 'apikey';
            proxy_redirect off;
        }
}

Bonus question

Is my way of adding the client second key valid?

add_header 'clientapikey' $clientapikey;

I will need in the future to create a location for each of my client, each with different clientapikey to identify them on the remote API.