The MSDN sample you referenced add images to the ImageList using the file extension as image key.
Only an image at a time can be associated to a key, thus if you didn't changed this behavior, maybe you have to modify that line of code
imageList1.Images.Add(file.Extension, iconForFile);
should be changed into
imageList1.Images.Add(file.FullName, iconForFile);
or something more convenient.
the same applies to
item.ImageKey = file.Extension;
that should be changed into
item.ImageKey = file.FullName;
Regards,
Daniele.