I want to edit a CAD document with IOM.dll in C# and put a thumbdail using a .jpg file but it is not putting the picture. How can I do it?

I have searched the forum before and people talk about Rons document. However I can't find it.
My actual code goes below, it is working for all other properties, except for the thumbdail:
itemX.setAction("edit");
string DateAndTime = DateTime.UtcNow.ToString("dd/MM/yyyy HH\\:mm\\:ss");
string AUX = itemX.getProperty("last_change_time");
itemX.getType();
itemX.setProperty("last_change_time", DateAndTime);
itemX.setFileProperty("native_file", NewFileNameAndPath);
itemX.setFileProperty("thumbnail", NewPicNameAndPath);
itemX.setProperty("description", "Hello World Agostinho Matos YY");
itemX.apply();