title: add css class to image attached in all the posts?
body: <p>In wordpress Currently by default image attachment have similar classes</p>
  
  <pre><code>&lt;img class="size-full wp-image-8996" src="https://nepaltimes.net/wp-content/uploads/2018/04/31043884_1792179470828697_8330507756888915968_n.jpg"&gt;
  </code></pre>
  
  <p><strong>What i am trying here to add a class as img-fluid to all the attachments in posts not the thumbnails.</strong></p>
  
  <pre><code>&lt;img class="img-fluid size-full wp-image-8996" src="https://nepaltimes.net/wp-content/uploads/2018/04/31043884_1792179470828697_8330507756888915968_n.jpg"&gt;
  </code></pre>
  
  <p>How could it be done?<br>
  Any Idea will be appreciated!
  Please help me</p>