Files with mdml extension are known as Markdown lightweigh markup language files.
MDML file extension - Markdown markup language
What is mdml file? How to open mdml files?
File type specification:
The mdml file extension is a Markdown lightweight markup language, originally created by John Gruber and Aaron Swartz, which aims for maximum readability and "publishability" of both its input and output forms, taking many cues from existing conventions for marking up plain text in email. Markdown converts its marked-up text input to valid, well-formed XHTML and replaces left-pointing angle brackets ('<') and ampersands with their corresponding character entity references. Markdown was originally implemented in Perl by Gruber, but has since been re-implemented by others in a variety of programming languages. It is distributed under a BSD-style license and is included with, or available as a plugin for, several content-management systems.