C# 下载文本文件乱码

if (File.Exists(filePath))
{
    FileInfo fi = new FileInfo(filePath);
    context.Response.Clear();
    context.Response.ClearHeaders();
    context.Response.Buffer = true;
    context.Response.Charset = "GB2312";
    //判断是否是IE浏览器
    if (explorerName.Contains("IE"))
        //是IE就对字符串编码
        context.Response.AppendHeader("Content-Disposition", "attachment;filename=" + HttpContext.Current.Server.UrlEncode(Path.GetFileName(filePath)));
    else
        //不是IE就不编码(Firefo、Chrome编码后会显示乱码)
        context.Response.AppendHeader("Content-Disposition", "attachment;filename=" + Path.GetFileName(filePath));
    context.Response.AppendHeader("Content-Length", model.VideoSize.ToString());
    context.Response.ContentType = "application/octet-stream";
    context.Response.Filter.Close();
    context.Response.TransmitFile(filePath);
    context.Response.Flush();
    context.Response.End();
}