title: Default Value for ActiveStorage Rails 5.2.0
body: <p>I am using Rails ActiveStorage. I want that whenever the value of attachment (in my case image is null then replace it with "abc.png" which is present in assets folder..)</p>
  
  <p>This is what my model.rb file looks like but this code does not seem to work. I am looking for how to set default / nil value for avatar. </p>
  
  <pre><code>      has_one_attached :avatar #bot icon
        after_create_commit check_avatar(self)
        def check_avatar(self)
          if(!self.avatar.present?)
          {
              self.avatar = "abc.png"
          }
        end
  </code></pre>