  <IfModule mod_rewrite.c>
    RewriteEngine on
    RewriteCond %{REQUEST_URI} ^/(media|thumbnail)/
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteRule ^(.*)$ https://MY_CDN_ADDRESS/$1 [QSA,L]
  </IfModule>

Caddyserver

    @mediaPaths {
        path /media/* /thumbnail/*
    }

    handle @mediaPaths {
        @notStatic not file
        redir @notStatic https://MY_CDN_ADDRESS/{path}
        file_server
    }

Nginx

    location ~ ^/(media|thumbnail)/ {
        try_files $uri @cdn;
    }
    location @cdn {
        return 302 https://MY_CDN_ADDRESS$request_uri;
    }