<form id="form1" runat="server">
 <div>
 <asp:FileUpload ID="FileUpload1" runat="server" />
 <asp:Button ID="Button1" runat="server" Text="上传" Width="54px" OnClick="Button1_Click" />
 <asp:Label ID="Label1" runat="server" Text="" Style="color: Red"></asp:Label>
 <asp:Image runat="server" ID="Image1" Style="z-index: 102; left: 20px; position: absolute;
  top: 49px" Width="73px" />
 </div>
 </form>

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

namespace Web.File
{
 public partial class WebForm1 : System.Web.UI.Page
 {
 protected void Page_Load(object sender, EventArgs e)
 {

 }
 #region 文件上传
 /// <summary>
 /// 文件上传
 /// </summary>
 protected void Button1_Click(object sender, EventArgs e)
 {
  if (FileUpload1.FileName == "")
  {
  this.Label1.Text = "上传文件不能为空";
  return;
  }

  bool fileIsValid = false;
  //如果确认了上传文件，则判断文件类型是否符合要求 
  if (this.FileUpload1.HasFile)
  {
  //获取上传文件的后缀 
  String fileExtension = System.IO.Path.GetExtension(this.FileUpload1.FileName).ToLower();
  String[] restrictExtension = { ".gif", ".jpg", ".bmp", ".png" };
  //判断文件类型是否符合要求 
  for (int i = 0; i < restrictExtension.Length; i++)
  {
   if (fileExtension == restrictExtension[i])
   {
   fileIsValid = true;
   }
   //如果文件类型符合要求,调用SaveAs方法实现上传,并显示相关信息 
   if (fileIsValid == true)
   {
   //上传文件是否大于10M
   if (FileUpload1.PostedFile.ContentLength > (10 * 1024 * 1024))
   {
    this.Label1.Text = "上传文件过大";
    return;
   }
   try
   {
    this.Image1.ImageUrl = "~/File/" + FileUpload1.FileName;
    this.FileUpload1.SaveAs(Server.MapPath("~/File/") + FileUpload1.FileName);
    this.Label1.Text = "文件上传成功!";
   }
   catch
   {
    this.Label1.Text = "文件上传失败！";
   }
   finally
   {

   }
   }
   else
   {
   this.Label1.Text = "只能够上传后缀为.gif,.jpg,.bmp,.png的文件";
   }
  }
  }
 }
 #endregion
 }
}

<!--因为FileUpload 控件上传最大为4M,如果要上传更大文件,改下maxRequestLength的大小-->
<configuration>
 <system.web>
 <compilation debug="true" targetFramework="4.0" />
 <httpRuntime requestValidationMode="2.0" maxRequestLength="10485760" executionTimeout="3600" appRequestQueueLimit="10000"/>
 </system.web>
</configuration>