title: AsyncFileUpload with HtmlEditorExtender throws validation error
body: <p>When I have an HTMLEditorExtender on the page (with HTML entered), and try to upload a file with the AJAX AsyncFileUpload control, I get a validation error <em>sometimes</em>:</p>
  
  <p><img src="https://i.stack.imgur.com/lEF1a.png" alt="enter image description here"></p>
  
  <p>I think I've narrowed it down - it only throws this error when the uploading is <em>after a postback</em> - with html entered in the TextBox.</p>
  
  <p>Here's an example:</p>
  
  <pre><code>&lt;asp:ScriptManager ID="ScriptManager1" runat="server"&gt;
  &lt;/asp:ScriptManager&gt;
  &lt;asp:Button ID="Button1" runat="server" Text="PostBack" /&gt;
  &lt;asp:TextBox ID="txtBannerHTML" runat="server" Height="130px" Width="100%" TextMode="MultiLine" /&gt;
  &lt;asp:HtmlEditorExtender ID="txtBannerHTML_HtmlEditorExtender" runat="server" DisplaySourceTab="True"
      Enabled="True" TargetControlID="txtBannerHTML"&gt;
  &lt;/asp:HtmlEditorExtender&gt;
  &lt;asp:AsyncFileUpload ID="AsyncFileUpload3" runat="server" /&gt;
  </code></pre>
  
  <p>To repro:</p>
  
  <ol>
  <li><p>Enter html in the Textbox.</p></li>
  <li><p>Hit the postback button.</p></li>
  <li><p>Try uploading an image.</p></li>
  </ol>
  
  <p>I do have the sanitizer enabled on the HTMLEditorExtender. </p>
  
  <p>Can anyone repro this?</p>
  
  <p>How do I get the two controls to work together?</p>