Set all non public api endpoints to be text/plain content type instead of json because the json has to be decrypted.

database/getMarketplaceIconData.php

@@ -1,6 +1,6 @@
 <?php
 require __DIR__ . '/../incl/util.php';
-setJsonHeader();
+setPlainHeader();
 checkClientDatabaseVersion();
 
 $post = getPostData();