.MDOWN File Extension
What is an MDOWN file?
Text file written using the Markdown language, a textual markup language used for creating documentation; saves plain text as well as markups that specify how to stylize and format the text (i.e., indentations, italics, headers, etc.); often used for documenting source code for software development projects.
Software development projects may include a file named README.mdown that contains the readme for the project.