浏览器下载Excel,直接打开显示乱码...

情景：

浏览器中点击下载文件有两个选项:[打开][下载]

[打开]之后,提示["文件.xlsx"的文件格式和扩展名不匹配。文件可能已损坏或不安全。除非您信任其来源，否则请勿打开。是否仍要打开它？]，点击[是],看到Excel乱码..
[下载]之后,再打开,会提示[发现"文件.xlsx"中的部分内容有问题。是否让我们尽量尝试恢复？如果您信任工作簿的源，请单击"是"]

扩展：工作簿的"源"，是将"文件.xlsx"，改后缀为"文件.zip"然后看到里面的xml(sharedStrings.xml) ？

原来代码以及下载情景：

        byte[] streamData = getByte("file.xlsx");
            Response.Charset = "UTF-8";
            Response.ContentEncoding = System.Text.Encoding.GetEncoding("UTF-8");
            Response.ContentType = "application/vnd.ms-excel";
            Response.AddHeader("Content-Disposition", string.Format("attachment,filename={0}", out_file_name));
            Response.AddHeader("Content-Length", streamData.LongLength.ToString());
            Response.BinaryWrite(streamData);
            Response.Flush();
            Response.End();

更改后代码及情景：

            Response.Charset = "UTF-8";
            Response.ContentEncoding = System.Text.Encoding.GetEncoding("UTF-8");
            Response.ContentType = "application/octet-stream";//改动
            Response.AddHeader("Content-Disposition", string.Format("attachment,filename={0}", out_file_name));
            Response.AddHeader("Content-Length", streamData.LongLength.ToString());

            Response.BinaryWrite(streamData);
            Response.Flush();
            Response.End();