我正在使用以下代码使用EPPlus向Excel工作表添加背景图像。但是保存的Excel没有任何背景图像。并且在使用在线Excel打开文件时，它说工作簿已损坏。

foreach (var file in Filelist)
{
    // Load workbook
    //var fileInfo = new FileInfo(@file);
    FileStream fs = new FileStream(file, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
    Bitmap bmp = new Bitmap("image.png");
    //ExcelPackage pkg = new ExcelPackage(fs);
    using (var package = new ExcelPackage(fs))
    {
        // Iterate over workbook sheets
        foreach (var sheet in package.Workbook.Worksheets)
        {
            sheet.BackgroundImage.Image = bmp;
            sheet.Protection.IsProtected = false;
        }
                   
        package.SaveAs(new FileInfo(@"New.xlsx"));
    }
    fs.Close();
}

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