added suggestion to match keys by file extension in the key

diff --git a/doc/todo/find__47__prefer_keys_by_file_extension_in_key.mdwn b/doc/todo/find__47__prefer_keys_by_file_extension_in_key.mdwn
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+Add [[preferred content expression|git-annex-preferred-content]] and [[matching option|git-annex-matching-options]] to match the file extension incorporated into a `*E` [[key|backends]], e.g. `keyext=.mp3` .  This would help address the limitation that `include=*.mp3` does not work with `--all` or `--unused`.