Excel file extension not valid - Microsoft Community
https://answers.microsoft.com/en-us/msoffice/forum/all/excel-file-extension-not-valid/af405961-6b55-4aba-83a3-ca503779314d
WebOct 24, 2017 · 1. Change the file extension of your Excel sheet. Sometimes, the problem is seen due to the unsupported file extension of your Excel file. Try renaming the file and change it from .xlsx to .xls or vice versa. 2. Open Excel in Safe mode. Here's how: a. Right-click the Start button (lower-left corner) in Windows, and click Run. b.
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