This script DOES import all of the file and it can be re-exported using Export Field Contents as expected.
My issue arose because I was was using the clipboard to open the file into the Preview app, FM puts a pict thumbnail into the clipboard not the contents of the container field, hence the confusion.
I have found my own solution by sleeping on it. Works wonders.
General comment I make here is that FM is a rather quirky app to develop with. Many things require work arounds and special fixes.