nginx mod_redirect if first part of uri is not a file

Question

I'm migrating from Apache to Nginx server and trying to figure out how to replace my .htaccess with nginx config.

In my rewrite I want to enable pretty links and need the server to check if the first part is not a file then redirect to index.php otherwise redirect to that file.

Example:

I have two PHP files in the root directory: index.php and files.php.

https://example.com/token/login ---> index.php/token/login
https://example.com/index.php/token/login ---> index.php/token/login //same as above

https://example.com/files.php/token/login ---> files.php/token/login

My current /default file is

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    server_name vpn.simpleinformatics.com www.vpn.simpleinformatics.com;
    return 301 https://$server_name$request_uri;
}
server {

    # SSL configuration

    listen 443 ssl http2 default_server;
    listen [::]:443 ssl http2 default_server;
    include snippets/ssl-vpn.simpleinformatics.com.conf;
    include snippets/ssl-params.conf;

    root /var/www/html;

    index index.php index.html index.htm index.nginx-debian.html;

    server_name vpn.simpleinformatics.com;

    location / {
        # try_files $uri $uri/ =404;
        # try_files $uri $uri/ /index.php;
        try_files $uri $uri/ /index.php$is_args$args;
    }

    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_pass unix:/run/php/php7.0-fpm.sock;
    }

    location ~ /\.ht {
        deny all;
    }
    location ~ /.well-known {
            allow all;
    }
    location /phpmyadmin {
        auth_basic "Admin Login";
        auth_basic_user_file /etc/nginx/pma_pass;
    }
}

This redirects all requests to index.php, even files.php/etc.. requests :(.

So how can I tell nginx to check if the first part of URI is a file, then do the redirect to that file.