diff --git a/layouts/shortcodes/video.html b/layouts/shortcodes/video.html
index b516252..984e2a9 100644
--- a/layouts/shortcodes/video.html
+++ b/layouts/shortcodes/video.html
@@ -7,7 +7,7 @@
 - height of player (in pixels)
 - preload (auto | metadata | none)
 
-!IMPORTANT: use starting / in file source: '/video/my-video'
+!IMPORTANT: use starting / in file source: '/video/my-video.mp4'
 -->
 
 <video width="{{ index .Params 1 }}" height="{{ index .Params 2 }}" preload="{{ index .Params 3 }}" controls>