Why is it when I save in Numbers, the file extension is .numbers-tef ?
When you save a document in Numbers you may see that the file extension is .numbers-tef rather than the .numbers you may be use to.
This is the extension for Numbers documents when you save them to iCloud for use on an iOS device, or another Mac.
If you change the folder to Documents you will see the file extension revert back to .numbers.
To avoid seeing this you need to stop using iCloud.