Excel cannot be open the file 'filename.xlsx' because the file format or file extension is not valid - Microsoft Community Hub
Excel cannot be open the file 'filename.xlsx' because the file format or file extension is not valid - Microsoft Community Hub
![Deciding between standard xlsx files and strict xlsx files. | FlexCel Studio for the .NET Framework documentation Deciding between standard xlsx files and strict xlsx files. | FlexCel Studio for the .NET Framework documentation](https://doc.tmssoftware.com/flexcel/net/images/save-as-strict-xlsx.png)