为什么只执行else语句？

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 FileUpload : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        bool filesValib = false;
        if (this.FileUpload1.HasFile)
        {
            string fileExtension = (this.FileUpload1.FileName).ToLower();
            string[] restricExtension = { ".gif", ".jpg", ".bmp", ".png" };
            for (int i = 0; i < restricExtension.Length; i++)
            {
                if (fileExtension == restricExtension[i])
                {
                    filesValib = true;
                }
            }

            if (filesValib == true)
            {
                try
                {
                    this.Image1.ImageUrl = "~/images/"+FileUpload1.FileName;
                    this.FileUpload1.SaveAs(Server.MapPath("~/images/") + FileUpload1.FileName);
                    this.Label1.Text = "文件上传成功";
                    this.Label1.Text += "<br/>";
                    this.Label1.Text += "<li>" + "源文件路径：" + this.FileUpload1.PostedFile.FileName;
                    this.Label1.Text += "<br/>";
                    this.Label1.Text += "<li>" + "文件大小：" + this.FileUpload1.PostedFile.ContentLength + "字节";
                    this.Label1.Text += "<br/>";
                    this.Label1.Text += "<li>" + "文件类型:" + this.FileUpload1.PostedFile.ContentType;
                }

                catch
                {
                    this.Label1.Text = "文件上传不成功！";
                }

                finally
                {

                }
            }
            else

            {
                this.Label1.Text = "只能够上传后缀名为jpg, gif, png, bmp的文件";
            }
        }
    }
}