There is only one file type that uses the .MEL extension:
Maya Embedded Language File
Script file containing code written in the Maya Embedded Language (MEL); developed for Maya 3D modeling and animation software; can be used to create macros and custom actions that speed up repetitive tasks.
MEL scripts can also be used to customize Maya's user interface. They may also call Python (.PY) code, which is the other scripting language supported by Maya.
Updated: December 4, 2014