Files with hmdl file extension might be found as 3D graphics files in HUMUS format.
There is one other file type using
the HMDL file extension!
.hmdl - Sega CANVAS Engine model
HMDL file extension - Humus Model data
What is hmdl file? How to open hmdl files?
File type specification:
The hmdl file extension is related to a custom-made file format created by Emil Persson, mostly known as "Humus".
Seems like a simple 3D graphics format used in his own projects.