Add workaround for matrix-org/synapse#15885.

XSS? What XSS? We host one static page on the domain, who cares

nixos/configuration.nix

@@ -174,6 +174,7 @@ in {
             proxy_set_header X-Forwarded-For $remote_addr;
             proxy_set_header X-Forwarded-Proto $scheme;
             proxy_set_header Host $host;
+            proxy_hide_header Content-Disposition; # matrix-org/synapse#15885
             proxy_buffering off;
           '';
         };