This is a text (.txt) file.

In Windows, every file must have an extension, e.g. 
- .txt for text files
- .py for python files
- .r for R files
- .zip for zip-compressed files

That is not a requirement on Mac or Linux. 
On those, a file namded `a` is just fine.
As a kindness to our friends in Windows-land, we generally 
inlcude extensions. It also helps us when looking at things with `ls`.