Code
<%@ WebHandler Language="C#" Class="Download" %>

using System;
using System.Web;

public class Download : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        string fileserverpath = context.Server.UrlDecode(context.Request.QueryString["file"]);
        System.IO.FileInfo fi = new System.IO.FileInfo(fileserverpath);
        fi.Attributes = System.IO.FileAttributes.Normal;
        System.IO.FileStream filestream = new System.IO.FileStream(fileserverpath, System.IO.FileMode.Open);
        long filesize = filestream.Length;
        int i = Convert.ToInt32(filesize);
        string p = fileserverpath.Substring(fileserverpath.LastIndexOf("/") + 1);
        string strTemp = System.Web.HttpUtility.UrlEncode(p, System.Text.Encoding.UTF8);//解决文件名乱码
        context.Response.ContentType = "application/octet-stream";
        context.Response.AddHeader("Content-Disposition", "attachment; filename=" +strTemp);
        context.Response.AddHeader("Content-Length", filesize.ToString());
        byte[] fileBuffer = new byte[i];
        filestream.Read(fileBuffer, 0, i);
        filestream.Close();
        context.Response.BinaryWrite(fileBuffer);
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
}