Whenever I try to save a spreadsheet as 'CSV (Comma delimited) (*.csv)' it appears to save itself as a CSV but has not actually converted the file type.
As a test, to make sure I wasn't going crazy, I saved a self-created Excel spreadsheet with multiple tabs and formatting as a CSV and there was no "may contain features that are not compatible" error, it just saved. If I navigate to where the file is, I can see the file extension and it definitely saves '.csv'. I right-click to check the Properties of the file, and look at the Details and it confirms that the file type is a 'Microsoft Excel Comma Separated Values...'.
However, when I go to open the file it warns me that the file "is in a different format than specified by the file extension" and the file retains the multiple tabs and formatting.
This error has only just started occurring.
I'm using Excel 2010, 32-bit.