Apparently it seems pretty common for subtitles to be put in a
subdirectory called "Subs", rather than in the same directory as the
media file itself.
This covers that usecase, by checking the returned listing for a
directory called "Subs" to exist.
If it does, its child nodes are added to the list before they're being
passed to mediaWithResources, allowing these subtitles to be discovered
automatically.
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