my $ct = $which eq 'html' ? 'text/html' : 'text/plain';
    return $ct{$ct} if exists $ct{$ct};

    # Fall back to plain text or HTML
    return $ct{'text/plain'} if exists $ct{'text/plain'};
    return $ct{'text/html'}  if exists $ct{'text/html'};

    # Fall back to whatever's first
    return $parts[0];