Own WebControl - FileUploader.HasFile is False

Hello,
I have a problem in Writing a own WebControl.
I have  Control with a asp:FileUpload in it. After Postback, hasFile from FileUpload is always false.
Why??
 
 
1 public class FileUploader : UserControl
2 {
3 #region Attributes
4
5 private readonly string FILE_LAYER_STYLE = "position: absolute; top: 0px; left: 0px; z-index: 1;";
6
7 private readonly string FILE_UPLOAD_ID = "fileUploadField";
8 private readonly string FILE_UPLOADER_TOOLTIP = "Bitte wählen Sie ihr ...