filemanager: fix sending of double header

we should not proceed to notFoundHandler if proxy handled it just fine

src/routes/filemanager.js

@@ -38,7 +38,7 @@ function proxy(kind) {
         const fileManagerProxy = middleware.proxy(proxyOptions);
 
         fileManagerProxy(req, res, function (error) {
-            if (!error) return next();
+            if (!error) return; // note: response was already sent by proxy by this point
 
             if (error.code === 'ECONNREFUSED') return next(new HttpError(424, 'Unable to connect to filemanager server'));
             if (error.code === 'ECONNRESET') return next(new HttpError(424, 'Unable to query filemanager server'));