Sorry for my late reply; very busy here at the moment. But we're getting closer... The files are uploaded, but no extension is given to the file, so the file is renamed like "2009452523".
Not sure if the error has something to do with not getting a thumbnail of the pdf -could be just be an icon or something- or that that problem lays within the missing extension.

<?php 
//Move to directory
if ($isAddionalAcceptedFile) {
   unset($errors); 
   @move_uploaded_file($_FILES['probe']['tmp_name'], $uploaded_images_path.$_FILES['probe']['name']) or $errors[] = 'upload_error '.$uploaded_images_path.$_FILES['probe']['name'];
 
} ?>