nginx .conf, download mp4 file only if there's a specific word in a query param

i have this code:

map $arg_dl $mimetype {
            1       application/octet-stream;
            default video/mp4;
    }

    server {
            listen          80;
            server_name     example.com www.example.com;
            root            /usr/share/nginx/html;
            index           index.html;

            location ~ \.mp4$ {
                    types {
                            $mimetype mp4;
                    }
                    default_type $mimetype;
            }

            error_page 404 /404.html;

the problem is that if I insert in the link of the video .mp4? dl = 1 I download the video correctly, but if I open the link of the video with url .mp4 or .mp4? dl = gfgfgf anyway the browser downloads the video. how can i solve? I need to download the video only if there is a one in the query and if there are other parameters to view the video in streaming. how can I do? Thank you

Clearly you cannot set types and default_type within an if block or with a mapped variable. One solution is to use a special URI in a location marked internal to handle the specific MIME type you require.

server {
    ...
    root /usr/share/nginx/html;

    location ~ \.mp4$ {
        if ($arg_dl) { rewrite ^ /download$uri last; }
        ...
    }
    location ^~ /download/ {
        internal;
        alias /usr/share/nginx/html/;
        types {}
        default_type application/octet-stream;
        expires -1;
    }
    ...
}

The ^~ modifier avoids ambiguity with other regular expression locations. The internal prevents /download being accessed directly. The location and alias should both end with a / or neither end with a /.