I'd like to report that as of ArchiCAD 18 (v7007), with our firm running x64 Windows 7 Ultimate, -Project preview images will work for icons so long as your saved ArchiCAD file types are .pln
Saving out a mod file for something like a detail library of separate detail files and changing the extensions from .mod to .pln will activate the saved project preview you created almost immediately, if not after a window refresh if on a server over a network.
Advantage: You keep the small file size a .mod would make, and you get the icon for reference in thumbnail view in File Explorer. Likewise, overwriting the file and maintaining its .pln extension will not increase the file size to that of a usual .pln
Has anyone figured out if this is resolved for ArchiCAD 19? -Our office has been trying to find a good time to upgrade, which is always a pain with teamwork in the mix.