Use correct headers for json data

It was previously not marked as such what could potentially lead to
browsers doing some autodetection.

Signed-off-by: Michal Čihař <michal@cihar.com>

file_echo.php

@@ -67,12 +67,16 @@
 
 } else if (isset($_REQUEST['monitorconfig'])) {
     /* For monitor chart config export */
-    PMA_downloadHeader('monitor.cfg', 'application/force-download');
+    PMA_downloadHeader('monitor.cfg', 'application/json; charset=UTF-8');
+    header('X-Content-Type-Options: nosniff');
+
     echo urldecode($_REQUEST['monitorconfig']);
 
 } else if (isset($_REQUEST['import'])) {
     /* For monitor chart config import */
-    header('Content-type: text/plain');
+    header('Content-Type: application/json; charset=UTF-8');
+    header('X-Content-Type-Options: nosniff');
+
     if (!file_exists($_FILES['file']['tmp_name'])) {
         exit();
     }