I get a message "Excel cannot open the file Content.xlsx because the file format of file extension is not valid. Verify that the file has not been corrupted and that the file extention matches the format of the file."

I have "lost" several Excel files recently and cannot figure out if I can recover them. 

Any help out there?

Hi keyman,

Based on your error message, did this issue only occur in a particular file? Or did it occur with all shared excel files?

The reasons of getting this error:
1. The file is not compatible with the version of Excel that is used.
2. The file has got corrupt.

If this issue only occurred in a particular file you can try to repair this file.

Sometimes, the problem is seen due to the unsupported file extension of your Excel file. To check the support of xlsx extension, go to Default Programs-Search Programs and Files- Set Your Default Programs- Select Excel- Choose Defaults For This Program. Check if 'xlsx' extension is included there or not. If not, then rename this file and change '.xlsx' to '.xls'.

You can also refer to these articles about this error:

https://support.microsoft.com/en-us/kb/928979

http://blogs.technet.com/b/wordonenotesupport/archive/2013/09/09/quot-cannot-open-the-file-because-the-file-format-or-extension-is-invalid-quot-opening-office-files.aspx

http://blogs.technet.com/b/the_microsoft_excel_support_team_blog/archive/2013/09/07/quot-cannot-open-the-file-because-the-file-format-or-extension-is-invalid-quot-opening-office-files.aspx

Hope it's helpful.

Regards,

Emi Zhang
TechNet Community Su