Since OpenSCAD doesn't distribute with additional image files, this means a new icon needs to be built into the .exe. This is accomplished by adding a line to the end of openscad_win32.rc file consisting of something like the following:
IDI_ICON2 ICON DISCARDABLE "icons/openscad_doc.ico"
which will get built into the exe as icon index 1 (icon index 0 is the current app icon.)
In the cmdline and gui functions in openscad.cc I added code to set the HKEY_CURRENT_USER/Software/Classes/OpenSCAD_File/DefaultIcon/Default value to the icon index 1 of openscad.exe
modified: openscad_win32.rc
modified: src/openscad.cc
