Whether STL https://en.wikipedia.org/wiki/STL_(file_format) is an "open format" (i.e. compatible with publication and redistribution under open licenses without restriction) isn't clear. The specification of the format was published in a book in October 1989: http://www.fabbers.com/tech/STL_Format
It is unclear whether the format is subject to patents.
The owner of the book's rights is likely to still be 3D Systems, a publicly traded US company: https://en.wikipedia.org/wiki/3D_Systems
We need to answer whether the format can be freely used on Wikimedia Commons in the same manner we do for all the formats we currently support.