added constants.php file and implemented constants for file system modes
diff --git a/user_guide/changelog.html b/user_guide/changelog.html
index 4e3ca7c..af51c0e 100644
--- a/user_guide/changelog.html
+++ b/user_guide/changelog.html
@@ -64,6 +64,7 @@
     	<ul>

     		<li>Added 'application/vnd.ms-powerpoint' to list of mime types.</li>

     		<li>Added 'audio/mpg' to list of mime types.</li>

+			<li>Added new user-modifiable file constants.php containing file mode constants</li>

     	</ul>

     </li>

 	<li>Libraries