in your page load method do as below
protected void Page_Load(object sender, EventArgs e)
{
Page.Form.Attributes.Add("enctype", "multipart/form-data");
ScriptManager scriptManager = ScriptManager.GetCurrent(this);
if (scriptManager != null)
{
scriptManager.RegisterPostBackControl(btnupload);
}
}
remove the Triggers from aspx page. do it from the code behind