diff --git a/boards/models/attachment/viewers.py b/boards/models/attachment/viewers.py
--- a/boards/models/attachment/viewers.py
+++ b/boards/models/attachment/viewers.py
@@ -11,6 +11,7 @@ FILE_TYPE_SVG = 'svg'
 FILE_TYPES_AUDIO = (
     'ogg',
     'mp3',
+    'opus',
 )
 
 PLAIN_FILE_FORMATS = {