文件上传问题！

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;public partial class 上传文件 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        lblMessage.Visible = false;
    }
    protected void btn_Click(object sender, EventArgs e)
    {
        lblMessage.Visible = true;
        string strPath = "";
        strPath = Server.MapPath("~/file/");        try
        {
            if (this.upload.HasFile)
            {
                if (this.upload.PostedFile.ContentLength > 4194304)
                {
                    this.lblMessage.Text = "对不起！上传文件的大小不能超过4MB";
                    return;
                }
                string strExtion = System.IO.Path.GetExtension(this.upload.FileName);
                if (strExtion.ToLower() != ".mp3" && strExtion.ToLower() != ".wav")
                {
                    this.lblMessage.Text = "对不起！只能上传.mp3或.wav的音频文件！";
                    this.upload.Focus();
                    return;
                }
                this.upload.PostedFile.SaveAs(strPath + this.upload.FileName);
                this.lblMessage.Text = "恭喜！上传文件成功！";
            }
            else
            {
                this.lblMessage.Text = "请选择上传文件！";
                return;
            }        }
        catch
        {
            this.lblMessage.Text = "上传文件失败！";
            return;
        }
    }
}请问这段代码中的 if (this.upload.PostedFile.ContentLength > 4194304)
                {
                    this.lblMessage.Text = "对不起！上传文件的大小不能超过4MB";
                    return;
                }
怎么不执行啊？谢谢！这段代码还有哪些问题啊？非常感谢！