Don't bother with this request; I've worked it out.

In case it is ever asked by another user the solution is:

From the "Tools" main menu item select "Properties for All Confogurations".

From the "All Confogurations" dialogue select the "Association" tab.

Tick the "Enable Association" box.

Bingo! Em now detects various file types.

Regards,
Peter

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There is nothing more difficult to take in hand, more perilous to conduct, or more uncertain in its success, than to take the lead in the introduction of a new order of things -- Niccolo Machiavelli, The Prince, ch. 6