If you check the file available for the MTH 20-3519 set, you'll find it's a .ZIP file and not an .MTH file.
Inside that ZIP file is two files, one is the .MTH sound file and the other is the ZIP file that contains all the chain files.
The sound file is loaded just like any other sound file. The chain file ZIP is used with the DCS Loader's FLASH upload feature to load the chain files for the PS/2 boards.
Chain files contain much of the control information that used to all be either in the sound file or the embedded firmware that the end user couldn't access. Stuff like gearbox ratio, lighting options, etc. are all in the chain file.