Add some extensions

[linebreaker]

Added a few common (relatively) 3D / architecture file formats.
Surprised you haven't listed .unicorn
After all, they are "binary" ;)

[sindresorhus]

For reference, can you list here exactly what the extensions are for?

[linebreaker]

1. .3dm
   Acronym for 3d Model
   The 3dm format is native to Rhino3D. Since McNeel and Associates have released NURBs (non-uniform rational bezier) specifiaction with OpenNURBs, the 3dm file format has become normative for NURBs files.
2. .gh
   Acronym for Grasshopper
   The GH format represents a binary grasshopper file, which is used in association with the above filetype (3dm) in programming environments, most often, but not exclusively, in Rhino3D's grasshopper, or Python sdks.
3. .stl
   Acronym for stereolithography
   The standard file format for cut-files, 3d print files, or other 3d data that does not otherwise conform to being described in terms of 'surfaces' or 'points' exclusively.
4. .wrm
   Acronym (potentially) for WiNRADiO
   WRM files are generic binary files that have been used by applications that do not have a specialized or proprietary binary format (this is distinguished from portability and usability, since there is no guarantee whatsoever that one application's file can be utilized by another application, and likely isn't). Common especially with backward compatibility applications since wrm has existed for a very, very long time.