YII2 How to add custom validation rule for file type

I am using :

[['avatar'], 'file', 'skipOnEmpty' => true, 'message'=>'png,jpg,jpeg files only', 'extensions' => 'png, jpg ,jpeg','on'=>'update']

It shows me validation message : "Only files with these extensions are allowed: png, jpg, jpeg." But i want validation message like: "png,jpg,jpeg files only". How can i do that ?