Specify the client max body size in the front, related to #371

(cherry picked from commit 6c56c8e298)

core/nginx/conf/nginx.conf

@@ -78,6 +78,7 @@ http {
         rewrite ^({{ WEB_WEBMAIL }})$ $1/ permanent;
         rewrite ^{{ WEB_WEBMAIL }}/(.*) /$1 break;
         include /etc/nginx/proxy.conf;
+        client_max_body_size 30M;
         proxy_pass http://$webmail;
       }
       {% endif %}