Response.WriteFile inside UpdatePanel

When I download file using Response.WriteFile inside UpdatePanel, I will get "Sys.WebForms.PageRequestManagerParseErrorException"

How can I avoid of the ParseErrorException using Response.WriteFile or Response.TransmitFile? I need to download some files and I don't want to use links.

Thanks in advance!

ASPX:
    <img src="loading.gif" />
    <asp:Button ID="btnSelectName" runat="server" Text="Select name of the picture" 
        onclick="btnSelectName_Click" />   
    <asp:UpdatePanel ID="UpdatePanel1" runat="server" Visible ="false">
        <ContentTemplate>
            <asp:TextBox ID="tbName" runat="server" Text="loading"/>
            <asp:Button ID="btnDownload" runat="server" Text="Download" 
                onclick="btnDownload_Click" />
        </ContentTemplate>
    </asp:UpdatePanel>
 
ASPX.CS:
    protected void btnSelectName_Click(object sender, EventArgs e)
    {
        UpdatePanel1.Visible = true;
    }
    protected void btnDownload_Click(object sender, EventArgs e)
    {
        Response.Clear();
        Response.ContentType = "image/GIF";
        Response.AppendHeader("content-disposition", "attachment;filename=" + tbName.Text + ".gif");
        //Response.TransmitFile("loading.gif");
        Response.WriteFile("loading.gif");
        Response.End();
    }

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