On my wiki this extension works with .stl and .json but not with .obj files. This might be because the MIME type isn't detected right, it shows as "MIME type: text/plain" for files where it shows "MIME type: application/obj" on another wiki that runs 3dAlloy.
See https://wiki.squirrelslair.ca/index.php?title=Mediawiki_Extension_3DAlloy/OBJ_file_problems
reported also as https://github.com/dolfinus/3DAlloy/issues/4