There is only one file type that uses the .MNL extension:
AutoCAD Menu LISP File
User interface file used by AutoCAD, an engineering and architecting design program; contains expressions that manipulate the user interface elements; written in Autodesk's proprietary AutoLISP language, which is based on LISP.
MNL files are used to customize the behavior of user interface elements in the AutoCAD workspace.
Updated: November 22, 2009