核心代码：

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DataSet ds = dBll.GetList("ID=" + ID);
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string docName = "a.doc";//文件名，
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        byte[] file = (byte[])ds.Tables[0].Rows[0]["FContent"]; //   "FContent"类型为image
        string Type = checktype(docName);
        Response.AddHeader("Content-Disposition", "attachment;filename=" + HttpUtility.UrlEncode(docName, System.Text.Encoding.UTF8).Replace("+", "%20"));
        Response.AddHeader("Content-Length ", file.Length.ToString());
        Response.ContentType = Type;
        Response.BinaryWrite(file);//可以下载二进制文件，如数据库存的image
        //Response.WriteFile(fliePath);可以下载路径文件
          Response.End();
        Response.Clear();
 
/// <summary>
/// 根据文件的扩展名来获取对应的“输出流的HTTP MIME“类型
/// </summary>
/// <param name="filename"></param>
/// <returns></returns>
private string checktype(string filename)
{
    string ContentType;
    switch (filename.Substring(filename.LastIndexOf(".")).Trim().ToLower())
    {
        case ".asf ":
            ContentType = "video/x-ms-asf ";
            break;
        case ".avi ":
            ContentType = "video/avi ";
            break;
        case ".doc ":
            ContentType = "application/msword ";
            break;
        case ".zip ":
            ContentType = "application/zip ";
            break;
        case ".xls ":
            ContentType = "application/vnd.ms-excel ";
            break;
        case ".gif ":
            ContentType = "image/gif ";
            break;
        case ".jpg ":
            ContentType = "image/jpeg ";
            break;
        case "jpeg ":
            ContentType = "image/jpeg ";
            break;
        case ".wav ":
            ContentType = "audio/wav ";
            break;
        case ".mp3 ":
            ContentType = "audio/mpeg3 ";
            break;
        case ".mpg ":
            ContentType = "video/mpeg ";
            break;
        case ".mepg ":
            ContentType = "video/mpeg ";
            break;
        case ".rtf ":
            ContentType = "application/rtf ";
            break;
        case ".html ":
            ContentType = "text/html ";
            break;
        case ".htm ":
            ContentType = "text/html ";
            break;
        case ".txt ":
            ContentType = "text/plain ";
            break;
        default:
            ContentType = "application/octet-stream ";
            break;
    }
    return ContentType;
}

 HttpUtility.UrlEncode(docName, System.Text.Encoding.UTF8).Replace("+", "%20"));这个可以解决中文文件名中包含空格的问题，UrlEncode编码后会把文件名中的空格转换中+（+转换为%2b），但是浏览器是不能理 解加号为空格的，所以在浏览器下载得到的文件，空格就变成了加号；
解决办法：UrlEncode 之后, 将 “+” 替换成 “%20″，因为浏览器将%20转换为空格