title: What is the purpose of Content-Type at response header?
body: <pre class="lang-js prettyprint-override"><code>const binary = xxxxx;
  res.set('Content-Type', 'image/jpg');
  res.send(binary);
  </code></pre>
  
  <p>The above is sample on returning an image, defining on express route. What I don't understand is that despite I change <code>Content-Type</code> to <code>application/json</code>, it seems to work too?</p>
  
  <pre class="lang-js prettyprint-override"><code>const binary = xxxxx;
  res.set('Content-Type', 'application/json'); //Still returning an image?
  res.send(binary);
  </code></pre>