diff --git a/lib/asciidoctor/chat-block-processor/extension.rb b/lib/asciidoctor/chat-block-processor/extension.rb
index a1f224d..b3ac977 100644
--- a/lib/asciidoctor/chat-block-processor/extension.rb
+++ b/lib/asciidoctor/chat-block-processor/extension.rb
@@ -6,7 +6,7 @@ class ChatBlock < Asciidoctor::Extensions::BlockProcessor
   named :chat
   on_context :example
   name_positional_attributes 'avatar', 'state'
-  default_attributes 'state' => 'default', 'avatarstype' => 'webp'
+  default_attributes 'state' => 'default', 'avatarstype' => 'avif'
 
   def process(parent, reader, attrs)
     block = create_block parent, :pass, nil, attrs, content_model: :compound