想请问一个基于WEB下的附件上传问题！

Question

private void Button1_Click(object sender, System.EventArgs e)
{
//检查上传文件不为空
if(file.PostedFile.FileName == "")
{
Response.Write("<script language='javascript' defer>");
Response.Write("alert('文件不能为空，请点击浏览按钮！')");
Response.Write("</script>");
}
else
{
string filename=(string)Session["ID"]+Path.GetFileName(file.PostedFile.FileName);
string path = Server.MapPath("Default.aspx").Replace("Default.aspx","")+"photos\\";
if(!Directory.Exists(path))
{
Directory.CreateDirectory(path);
}
path+=filename;
if(file.PostedFile!=null)
{  
try
{

if(file.PostedFile.ContentLength > 1048576)
{
Response.Write("<script language='javascript' defer>");
Response.Write("alert('文件不能超过1M！')");
Response.Write("</script>");
return;
}


string geshi = filename.Substring((filename.LastIndexOf('.')+1),3).ToLower();
if(( geshi == "gif") || ( geshi == "jpg" ) || ( geshi == "bmp" ))
{
file.PostedFile.SaveAs(path);
//ddlFile.Items.Add(new ListItem(filename));
lblFile.Text = filename;
returnvalue = filename;
fname.Text=file.PostedFile.FileName;
fenc.Text=file.PostedFile.ContentType ;
fsize.Text=file.PostedFile.ContentLength.ToString();

}
else
{

Response.Write("<script language='javascript' defer>");
Response.Write("alert('图片格式应为：gif、bmp、jpg。')");
Response.Write("</script>");
return;
}
//Response.Write(geshi); }
catch(Exception ex)
{
Response.Write(ex.Message);
}
}
}
}