Support webp format (#1785)

* consider webP as an input and output format, meaning a derivative from webp is still a webp

admin/include/image.class.php

@@ -47,6 +47,7 @@ class pwg_image
 
   function __construct($source_filepath, $library=null)
   {
+    global $conf;
     $this->source_filepath = $source_filepath;
 
     trigger_notify('load_image_library', array(&$this) );
@@ -58,7 +59,7 @@ class pwg_image
 
     $extension = strtolower(get_extension($source_filepath));
 
-    if (!in_array($extension, array('jpg', 'jpeg', 'png', 'gif')))
+    if (!in_array($extension, $conf['picture_ext']))
     {
       die('[Image] unsupported file extension');
     }
@@ -810,4 +811,4 @@ class image_gd implements imageInterface
   }
 }
 
-?>
+?>