add missing connect-src directives to example nginx conf

docs/example.nginx.conf

@@ -72,7 +72,7 @@ server {
     set $styleSrc   "'unsafe-inline' 'self' ${main_domain}";
 
     # connect-src restricts URLs which can be loaded using script interfaces
-    set $connectSrc "'self' https://${main_domain} $main_domain https://${api_domain} blob:";
+    set $connectSrc "'self' https://${main_domain} ${main_domain} https://${api_domain} blob: wss://${api_domain} ${api_domain} ${files_domain}";
 
     # fonts can be loaded from data-URLs or the main domain
     set $fontSrc    "'self' data: ${main_domain}";