title: If an <a> link is a PDF file, style accordingly
body: <p>I am working on a website for a client, who is an exam board. They have a number of content pages with summary details about their syllabuses, and need to link directly to PDF Syllabus files from within the text.
  There is also a page just for PDF files, and I am currently making those Custom Post Types in WordPress, but those have their own icon setup that I don't want being affected by the PHP rule that I'm about to describe.
  (This is more of a PHP question than a WordPress question, so don't worry.)</p>
  
  <p>Let's say I have a link, New Syllabus. With WordPress, one is unable to add a class to the link, without knowledge of HTML, which in the case of my client and its rather old-school employees, is non-existent. They will be shown how to add links to PDF files using the Add Media button in the WordPress post or page editor, which will simply create a link to the .pdf file. This issue here is that it's just a link, and the client wants users to immediately know that it's a PDF file, and not a page, to make it easier to find the relevant syllabus files. Therefore, I've proposed creating a rule that adds a .png icon next to any link that is linking to a .pdf file, and it'll change the link colour as well.</p>
  
  <p>Does anyone have any idea how I could achieve this? I've looked for WordPress plugins that do this automatically, but they're either outdated or hard to use (i.e., employees will need HTML knowledge to restyle the links.)</p>
  
  <p>I found this code snippet online, but don't know if it's the right sort of direction.</p>
  
  <pre><code>$whitelist = array(".pdf");
  
  foreach ($whitelist as $item) {
  
      if (preg_match("/$item\$/i", $_FILES['uploadfile']['name'])) {
  
      }
      else {
  
          redirect_to("index.php");
      }
  }
  </code></pre>
  
  <p>Thoughts?</p>