title: How to validate an Image
body: <p>I have <code>asp:FileUpload</code> field.</p>
  
  <p>I want to be sure it's an image, validate it with validators</p>
  
  <p>file FileUpload</p>
  
  <pre><code>&lt;asp:FileUpload ID="P1Image" runat="server" /&gt;
  </code></pre>
  
  <p>file Validator</p>
  
  <pre><code>&lt;asp:CustomValidator ID="CustomValidator1" ControlToValidate="P1Image" runat="server" ErrorMessage="Choose an image"&gt;&lt;/asp:CustomValidator&gt;
  </code></pre>
  
  <p>I can send it to JS function but what is the test I need to do?</p>