asp.net 下载图片

public class DownLoad : IHttpHandler
    {

        public void ProcessRequest(HttpContext context)
        {
            context.Response.ContentType = "text/plain";
            string fileName = context.Request.QueryString["filename"];
            //火狐浏览器支持中文传输，编码后反倒不认了，所以要判断排除火狐浏览器再编码
            string UserAgent = context.Request.ServerVariables["http_user_agent"].ToLower();
            if (UserAgent.IndexOf("firefox") == -)
            {//非火狐浏览器
                fileName = HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8);
            }
            context.Response.AddHeader("Content-Disposition", "attachment;filename=" + fileName);
            context.Response.WriteFile(context.Server.MapPath("Down/") + context.Request.QueryString["filename"]); //这里一定要未编码的文件名字。否则找不到文件
        }

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