title: file validation - content type or extension?
body: <p>I need to validate if a file is an image. 
  Should I check <strong>content type</strong> or <strong>extension</strong>? What is more safe / better?
  I think checking extension is better - what do you think?</p>
  
  <pre><code>string ext = System.IO.Path.GetExtension(fileName).ToLower();
  </code></pre>