The file format and extension of don't match. The file could be corrupted or unsafe

Question

I'm creating an Excel file with multiple sheets using DocumentFormat.OpenXml library from my C# code. The file gets created without any issue and it has all the required data but when I open it, I get this error pop-up:

I have noticed that this erroneous file has a type of XSLX file. If it is of the type Microsoft Excel Worksheet, strangely, it doesn't throw any error:

I'm just not getting what is wrong here an how I can fix it. Need suggestions, please. Below is my code snippet:

private static void WriteToExcelFile(HashSet<string> modules, Dictionary<string, HashSet<MyData>> dataDictionary)
        {
            if (!Directory.Exists(Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location), "Content\\MyDocument")))
                Directory.CreateDirectory(Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location), "Content\\MyDocument"));

            string fileFullname = Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location), "Content\\MyDocument", "MyExcel.xslx");
            SpreadsheetDocument spreadsheetDocument = SpreadsheetDocument.Create(fileFullname, SpreadsheetDocumentType.Workbook);

            WorkbookPart workbookpart = spreadsheetDocument.AddWorkbookPart(); // Add a WorkbookPart to the document
            workbookpart.Workbook = new Workbook();

            // Add Sheets to the Workbook  
            Sheets sheets = spreadsheetDocument.WorkbookPart.Workbook.AppendChild(new Sheets());

           // populate sheets
           .....

           spreadsheetDocument.Close();
}

Answer 1

The error comes from a wrong extension: it MUST be .xlsx And not .xslx