Patch security issue on empty salt

A configured salt is necessary for generating security keys
According to the comment above its definition, keeping it empty
was supposed to disable the feature. Instead, it kept it insecurely
enabled and used the empty salt, giving absolutely predictible
security keys considered as valid instead of not returning any
Note that commenting-out the define properly disabled this feature

This behavior is now fixed. If you do not update and have an empty
salt, either remove/comment the define, or set one immediately

Default allowed file extensions have also been edited, showing
regex are supported (which they are since the beginning) to make
sure users are aware of that possibility as it wasn't documented

Version bumped to 2.1.1

Author: Xenthys

sharexen.php

@@ -25,22 +25,23 @@
 ]);
 
 // Security keys salt - NEVER SHARE THIS
-// Format: define('SALT', 'randomstring');
 // Used to generate and compute security keys
 // Changing this will render all previously generated
 // deletion URLs invalid without any exception
-// Keep empty to disable this feature, only admins will
-// then be able to delete files without security keys
+// Keep as-is, set empty, or remove the define
+// to disable this feature, only admins will then
+// be able to delete files without security keys
 // Mandatory for having deletion URLs, set this to
 // a very long and random string of various characters
 // Random generator: https://bfnt.io/pwgen
-define('SALT', '');
+define('SALT', 'change-me');
 
 // List of allowed image extensions
 // Only put image extensions here unless
 // you edit the MIME_TYPE_REGEX option as well,
 // which is very discouraged for security reasons
-define('EXTS', ['png', 'jpg', 'jpeg', 'gif', 'webm', 'mp4']);
+// Regular expressions can be used as well here
+define('EXTS', ['png', 'jpe?g', 'gif', 'webm', 'mp4']);
 
 
 /* OPTIONAL CONSTANTS BELOW THIS LINE */
@@ -119,7 +120,7 @@
 \*****************************/
 
 
-define('VERSION', '2.1.0');
+define('VERSION', '2.1.1');
 define('SOURCE', 'https://github.com/Xenthys/ShareXen');
 
 $data = [
@@ -346,7 +347,7 @@ function error_die(&$data, $code, $reason = 'unknown_error', $debug = '')
 
 function retrieve_key($name)
 {
-	if (!defined('SALT'))
+	if (!defined('SALT') || !SALT || SALT === "change-me")
 	{
 		return false;
 	}