CMS Made Simple (CMSMS) Showtime2 File Upload RCE

This module exploits a File Upload vulnerability that lead in a RCE in
Showtime2 module ( user with "Use Showtime2" privilege could exploit the vulnerability.

The vulnerability exists in the Showtime2 module, where the class
"class.showtime2_image.php" does not ensure that a watermark file
has a standard image file extension (GIF, JPG, JPEG, or PNG).

Tested on Showtime2 3.6.2, 3.6.1, 3.6.0, 3.5.4, 3.5.3, 3.5.2, 3.5.1, 3.5.0,
3.4.5, 3.4.3, 3.4.2 on CMS Made Simple (CMSMS) 2.2.9.1