Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php       (revision 16607)
+++ wp-includes/post.php       (working copy)
@@ -3895,10 +3895,24 @@
 
        $ext = preg_match('/\.([^.]+)$/', $file, $matches) ? strtolower($matches[1]) : false;
 
-       $image_exts = array('jpg', 'jpeg', 'gif', 'png');
+       // allow adjustments to thumbnailable extensions
+        $image_exts = apply_filters(
+               'thumbnailable_extensions',
+               array('jpg', 'jpeg', 'gif', 'png')
+       );
 
-       if ( 'image/' == substr($post->post_mime_type, 0, 6) || $ext && 'import' == $post->post_mime_type && in_array($ext, $image_exts) )
+       // what do we know about the attachment?
+       $is_mime_image    = 'image/' == substr($post->post_mime_type, 0, 6);
+       $is_thumbnailable = in_array($ext, $image_exts);
+
+       // if the mime type has this as an image, then it's an image
+       if($is_mime_image)
                return true;
+
+       // if it's an otherwise imported image, but we accept the file extension...
+       if($is_thumbnailable)
+               return true;
+
        return false;
 }