Thank you everyone. I did a quick check and I can't get any of the solutions suggested by tincho, Bruce or cogier to work. As pointed out already, odt files are excluded and as it turns out, so too are Freeplane *.mm files. I didn't have time to explore further, but it seems files with extensions that share one of the characters in 'html' or 'txt" are also excluded.
Perhaps this is not such a simple question as originally thought. It will be a week or so before I have time to follow up.
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