Only the old Excel format (.xls) can be used as an attachment in posts. The newer Excel format (.xlsx) that actually came out almost 9 years ago in 2007 is listed as 'invalid'.

You might want to add .xlsx to the list...

 

Done

"Save as" XLS would also work but this gets current version working.

Thanks for the suggestion!

 

Yes, I had to convert a file I wanted to add to a post from the newer .xlsx format to the old .xls format so I could attach it. I figured you might want to add the newer .xlsx format, especially since it is now already 9 years.

Glad to help...