<asp:FileUpload ID="fuAttach" Multiple="Multiple" runat="server" Visible="False" />

'System.Web.UI.WebControls.FileUpload' does not contain a definition for 'PostedFiles' and no extension method 'PostedFiles' accepting a first argument of type 'System.Web.UI.WebControls.FileUpload' could be found (are you missing a using directive or an assembly reference?)

public class FileUpload : WebControl
{
    public FileUpload();

    public byte[] FileBytes { get; }
    public Stream FileContent { get; }
    public string FileName { get; }
    public bool HasFile { get; }
    public HttpPostedFile PostedFile { get; }

    protected override void AddAttributesToRender(HtmlTextWriter writer);
    protected internal override void OnPreRender(EventArgs e);
    protected internal override void Render(HtmlTextWriter writer);
    public void SaveAs(string filename);
}

<asp:FileUpload runat="server" ID="UploadImages" AllowMultiple="true" />

<div>
    <asp:FileUpload runat="server" ID="UploadImages" AllowMultiple="true" />
    <asp:Button runat="server" ID="uploadedFile" Text="Upload" OnClick="uploadFile_Click" />
    <asp:Label ID="listofuploadedfiles" runat="server" />
</div>

protected void uploadFile_Click(object sender, EventArgs e)
{
   if (UploadImages.HasFiles)
   {
       foreach (HttpPostedFile uploadedFile in UploadImages.PostedFiles)
       {
           uploadedFile.SaveAs(System.IO.Path.Combine(Server.MapPath("~/Images/"),
           uploadedFile.FileName)); listofuploadedfiles.Text += String.Format("{0}<br />", uploadedFile.FileName);
       }
   }
}

dynamic fileUploadControl = fileUpload1;
foreach(var file in fileUploadControl.PostedFiles)
{//do things here}