There is only one file type that uses the .MTHD extension:
MOPEKS Method File
File created by MOPEKS, a freeware program that generates multiple programs run by its script to use each other to solve problems; contains a method, which is played when clicked.
When you open the MOPEKS application for the first time, a window will open asking if you want to make the MTHD file associated with the MOPEKS program, click.
NOTE: Other MOPEKS associated files include .MPKT, .MSCT, and .MTXT.
Updated: July 28, 2019