DWGicon.dll is a Windows shell extension that enables you to identify the versio
n of a drawing just by looking at its icon in explorer.
All drawings show an icon with a revision number that corresponds to the AutoCAD revision this drawing has been saved as.
Revision up to R14 are red, from R2000 onwards blue. Password protected R2004, R2006 and R2007 drawings files are of the teal colour.
New drawings (later than R2007) will show a yellow star. If you see that, it is time to get a new version of DWGicon.dll.
If DWGicon doesn't recognise a file to be a valid drawing it will show a question mark. This indicates an invalid file as opposed to a new revision.