There is a bug report/ticket open:
You can go with that workaround till it is fixed:
1. Right click on the file > Properties
2. In the "general" tab click the "Unblock" button
3. Click "Apply"
4. Click "OK"
5. Double click the file to open it
6. Observe the dialog box that opens "The file format and extension of '[Filname].xls' don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. Do you want to open it anyway?"
7. Click yes
8. File opens
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