A file is in a different file format than its extension indicates
The file that you are trying to open in Microsoft Excel 2010 has a file name extension that does not match the actual file format of the file. For example, the file has an .xlsx file name extension but it really is a text file that should have a .txt file name extension.
Excel prompts you to verify that the file is not corrupted and that it originated from a source that you trust. Before you click Yes to open the file, you may want to consider the reasons why the file might have an incorrect file name extension.
For example, the file might have been:
Renamed inadvertently If you or someone you know renamed the file by accident so that it has an incorrect file name extension you can safely open the file. Then, save the file in the file format that you want with the appropriate file name extension.
Renamed intentionally If you or someone you know purposely renamed the file by using a different extension to ensure that Excel will recognize the file name extension, you can open the file. However, as a best practice, you should save the file with a file name extension that reflects its actual file format.
As a first step, however, it is important to decide whether or not you trust the source of the file. If you can't verify that the originator of the file is a trusted source, you should not open the file. A hacker (malicious user) might have renamed the file with the intent of misleading you to open it in Excel.
An issue with a file name extension might occur when a workbook that you are trying to open is unreadable and the contents no longer resemble the expected file format. If a workbook has become corrupted, you can try to repair it before you open it. For information about how to recover a corrupted workbook, see Repairing a corrupted workbook.
If you see this message when you try to open a workbook that a trusted source sent to you for review in an email message, you cannot open it from your email program. The person who sent it to you should resend the workbook in the Excel Binary Workbook (*.xlsb) or Excel 97-2003 Workbook (*.xls) file format. Alternatively, you can save the attached workbook on your computer, and then open it in Excel.