title: webpack: question mark after [ext] in config file
body: <p>webpack.config.js, under rules:</p>
  
  <pre><code>  {
      test: /\.(png|jpg|gif|svg)$/,
      loader: 'file-loader',
      options: {
        name: '[name].[ext]?[hash]'
      }
    }
  </code></pre>
  
  <p>what does <code>?[hash]</code> do?</p>