The newer xlsx file type is really just an XML file. CSV files can also be opened by Excel, and Excel will sometimes change the MIME type to it’s vendor-specific (vnd prefix) MIME type. Because you know…Microsoft.
You aren’t doing anything wrong. This is a larger issue in the tech world. This StackOverflow post shows that this question can have many answers.
If you want to be “more correct” with your CSV files, definitely always save them with a “text/csv” type. The data people in your life will thank you.