fix(filefield): do not assume index of files

whern adding, removing then adding again a file, indexes are somewhat consumed. Indes 0 will not exist in uploads list.

inc/field/filefield.class.php

@@ -129,8 +129,7 @@ public function moveUploads() {
          return;
       }
       $answer_value = [];
-      $index = 0;
-      foreach ($this->uploads["_$key"] as $document) {
+      foreach ($this->uploads["_$key"] as $index => $document) {
          $document = Toolbox::stripslashes_deep($document);
          if (is_file(GLPI_TMP_DIR . '/' . $document)) {
             $prefix = $this->uploads['_prefix_formcreator_field_' . $this->question->getID()][$index];