ckfinder browser displaying filetype icon instead of image t

First of all let's make sure that CKFinder is configured to display thumbnails (on the end-user side):
Click on Settings -> View and choose "Thumbnails", click "Close" to save settings.

On the server side, in configuration file (config.php) $config['Thumbnails']['enabled'] must be set to true.

Now open CKFinder and some tool to trace AJAX calls (HTTP requests), you should see something similar to:
http://www.example.com/ckfinder/core/co ... e=test.jpg

Open this in your browser and check what you see.
If you see a blank page instead of a thumbnail, set error_reporting to E_ALL and display_errors to On.
For example, add this code to config.php:

ini_set('display_errors', 1);
error_reporting(E_ALL);