上传图片文件并显示图片

protected void Button1_Click(object sender, EventArgs e)
{
     if (FileUpload1.PostedFile.FileName == "")
     {
         Page.ClientScript.RegisterStartupScript(Page.GetType(), "message", "<script language='javascript' defer>alert('文件名称不能为空！');</script>");
         return;
     }
     else
     {
         string filepath = FileUpload1.PostedFile.FileName;
         Response.Write("文件全路径" + filepath);
         string filename = filepath.Substring(filepath.LastIndexOf("\\") + 1);
         Response.Write("文件名称" + filename);
         string fileex = filepath.Substring(filepath.LastIndexOf(".") + 1);
         Response.Write("文件扩展名" + fileex);
         string serverpath = Server.MapPath("img/") + filename;
         Response.Write("文件存储全路径" + serverpath);
         if (fileex == "png")
         {
             FileUpload1.PostedFile.SaveAs(serverpath);
             Image1.ImageUrl = "img/" + filename;
             Page.ClientScript.RegisterStartupScript(Page.GetType(), "message", "<script language='javascript' defer>alert('上传成功！');</script>");
         }
         else
         {
             Page.ClientScript.RegisterStartupScript(Page.GetType(), "message", "<script language='javascript' defer>alert('文件类型错误！');</script>");
         }
     }
}