Replace is_file() with the faster file_exists()
(where it makes sense)

Also:
 - Implemented caching of configuration arrays for smileys, foreign characters and doctypes.
 - Implemented cascading-style loading of configuration files (except for library configs, DB and constants.php).
diff --git a/system/core/Common.php b/system/core/Common.php
index 90cc5b3..258cd49 100644
--- a/system/core/Common.php
+++ b/system/core/Common.php
@@ -316,11 +316,11 @@
 	{
 		static $_mimes = array();
 
-		if (is_file(APPPATH.'config/'.ENVIRONMENT.'/mimes.php'))
+		if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/mimes.php'))
 		{
 			$_mimes = include(APPPATH.'config/'.ENVIRONMENT.'/mimes.php');
 		}
-		elseif (is_file(APPPATH.'config/mimes.php'))
+		elseif (file_exists(APPPATH.'config/mimes.php'))
 		{
 			$_mimes = include(APPPATH.'config/mimes.php');
 		}