change:
<input name="file_tags[]" type="checkbox" id="file_tags_<?php echo $row_tags['tag'] ?>" value="<?php echo $row_tags['tag']; ?>" <?php if (in_array($row_WADAresources['file_tags'], $tags_selected)) { print('checked="checked"');} ?> />
to:
<input name="file_tags[]" type="checkbox" id="file_tags_<?php echo $row_tags['tag'] ?>" value="<?php echo $row_tags['tag']; ?>" <?php if (in_array($row_tags['tag'], $tags_selected)) { print('checked="checked"');} ?> />