[feature/avatars] Move list of supported formats to avatar driver class

Using the regex and turning it into an array if necessary seemed like the
cleanest approach to achieve this.

PHPBB3-10018

phpBB/includes/avatar/driver/driver.php

@@ -51,6 +51,11 @@ abstract class phpbb_avatar_driver implements phpbb_avatar_driver_interface
 	*/
 	protected $cache;
 
+	/**
+	* Regex for allowed avatar image extensions
+	*/
+	const REGEX_ALLOWED_EXT = 'gif|jpg|jpeg|png';
+
 	/**
 	* Construct a driver object
 	*