Hello nico change this to the form
From:
<form class="DetailsPage DetailsPage Compact_Default" id="Details_Compact_Default" name="Details_Compact_Default" method="post" action="<?php echo (htmlentities($_SERVER["PHP_SELF"], ENT_QUOTES)); ?>">
To
<form action="<?php echo (htmlentities($_SERVER["PHP_SELF"], ENT_QUOTES)); ?>" method="post" enctype="multipart/form-data" name="Details_Compact_Default" class="DetailsPage DetailsPage Compact_Default" id="Details_Compact_Default">
and test it again the form is missing enctype="multipart/form-data"