whitelist changes:

* add gif and jpg as allowed data images
* ensure that user controlled content fall only in the "data section" of the data URI (and does not intersect content-type definition in any way (best to be safe than sorry ;-)))
  "data section" as defined in: https://tools.ietf.org/html/rfc2397#section-3

Parsedown.php

@@ -91,7 +91,9 @@ class Parsedown
         'ftp://',
         'ftps://',
         'mailto:',
-        'data:image/png;',
+        'data:image/png;base64,',
+        'data:image/gif;base64,',
+        'data:image/jpg;base64,',
     );
 
     #