Those aren't dates. Those are strings. You need to put your dates into a proper format that is either sortable as strings or compare them as dates using DateTime()
:
usort($filesWithDates, function($a, $b) {
$date1 = DateTime::createFromFormat('d/m/Y', $a);
$date2 = DateTime::createFromFormat('d/m/Y', $b);
return $date1 > $date2;
});
foreach ($filesWithDates as $value) {
echo $value."<br/>";
}