file disappearing in upload file control on post back

Dear All,
I have a file upload control, a dropdown control(with post back enabled) and a button to upload the selected file in my .aspx page.
To avoid page flickering I am using an update panel in my master page.
The issue is after selecting a file to upload and when I select an item from the dropdown, the selected file in upload control disappears.
If I don t select item from dropdown, the selected file is shown as it is.
How can I rectify this problem?
I am putting the below code in my page load event
If Not IsPostBack Then
              Dim updatePanel As UpdatePanel = TryCast(Page.Master.FindControl("UpdatePanel1"), UpdatePanel)
              Dim trigger As UpdatePanelControlTrigger = New PostBackTrigger()
         & ...