I started to create markdown files and immediately stuck with a problem of saving my work with correct file extension.
Firstly, I did a mistake naming it a “markup” rather then a “markdown” when googling it. It is actually “Markdown“. It is a subset of “Markup” which is defined as “a way of providing functionality above plain text”.
Hence, the text with markdown formatting has an extension “.markdown“. There is a short version of it: “.md“. It seems to be more popular (for example: used by Github, Bitbucket, etc. by default), as it simply shorter. Both extensions are valid.