FileUpload in AJAX-enabled web applications

I would like to note that this article is not about the ability to upload files to the server without the postback. There are a lot of articles on this topic, just type "AJAX FileUpload" in any search engine and you'll get many examples. However with AJAX they actually have little in common, because the XMLHttpRequest does not support asynchronous uploading of files to the server, they are rather a variety of imitations, for example, using hidden IFRAME element. Nevertheless I want to emphasize that the article is not about that but about the standard FileUpload control.
There are two problems you might encounter when using it on UpdatePanel.
Problem 1
If the postback is caused by a control which lies on the UpdatePanel, the FileUpload is always empty when it come to the server, regardless whether a file has been selected or not.
Example:

<asp:UpdatePanel ID="UpdatePanel1" runat=server>
  <ContentTemplate>
     <asp:FileUpload ID="FileUpload1" runat=server />
     <asp:Button ID="btnUpload" runat=server Text="Upload" OnClick="btnUpload_Click"/>
  </ContentTemplate>
</asp:UpdatePanel>

Solution
As XMLHttpRequest does not allow to send files asynchronously, they have to be submitted in a common manner. This problem is well described around, it is solved by registration of the control that has to submit the form as a postback trigger (in the above example it is btnUpload button).

<asp:UpdatePanel ID="UpdatePanel1" runat=server>
  <ContentTemplate>
     <asp:FileUpload ID="FileUpload1" runat=server />
     <asp:Button ID="btnUpload" runat=server Text="Upload 2" OnClick="btnUpload_Click"/>
  </ContentTemplate>
  <Triggers>
      <asp:PostBackTrigger ControlID="btnUpload" />
  </Triggers>
</asp:UpdatePanel>

Problem 2
FileUpload does not work if it is loaded not on the initial page load but appears only after asynchronous update of the page part.
Example (pnlUpload panel is invisible in the beginning and is shown after clicking on btnShowFileUpload button):

<asp:UpdatePanel ID="UpdatePanel1" runat=server>
   <ContentTemplate>
        <asp:Button ID="btnShowFileUpload" runat=server Text="Show File Upload" OnClick="btnShowFileUpload_Click"/>
        <asp:Panel ID="pnlUpload" runat=server Visible="False">
           <asp:FileUpload ID="FileUpload1" runat=server />
           <asp:Button ID="btnUpload" runat=server Text="Upload" OnClick="btnUpload_Click"/>
       </asp:Panel>
   </ContentTemplate>
   <Triggers>
        <asp:PostBackTrigger ControlID="btnUpload" />
   </Triggers>
</asp:UpdatePanel>

.......................

protected void btnShowFileUpload_Click(object sender, EventArgs e)
{
    pnlUpload.Visible = true;
}

Solution
The problem is caused by the requirement that for the normal work of FileUpload the form should have enctype="multipart/form-data". Usually, it is set in overriden OnPreRender method of FileUpload control.

protected internal override void OnPreRender(EventArgs e)
{
    base.OnPreRender(e);
    HtmlForm form = this.Page.Form;
    if ((form != null) && (form.Enctype.Length == 0))
    {
        form.Enctype = "multipart/form-data";
    }
}

Although during asynchronous postback this code is also executed but the form is not updated. That is why it is required to set the form content type explicitly during the first page load, for example, in the Page_Load event handler of the page or a control where FileUpload is placed.

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
        this.Page.Form.Enctype = "multipart/form-data";
}

In case if this task is repeated in a few places you may do a simple control derived from FileUpload with overriden OnLoad method and use it.

public class CustomFileUpload : FileUpload
{
 protected override void OnLoad(EventArgs e)
 {
     base.OnLoad(e);

     if (!Page.IsPostBack)
         this.Page.Form.Enctype = "multipart/form-data";
 }
}