Hi Uncle Cheese!
I ran into this problem today and checked the older threads on the topic. I had a look at FileDataObjectManager and the change with newClassInstance was there. However I was still getting the error and when I looked at the stack trace it referred to ImageGalleryManager->saveImportForm so I went and had a look and ImageGalleryManager has the old code with just $file->ClassName = $this->fileClassName.
I replaced that with
if($file->ClassName != $this->fileClassName) {
$file = $file->newClassInstance($this->fileClassName);
}
and now everything works fine. This issue only happens with a few of the images on that installation so this is the first time this issue has popped up.