Hi, I am traing tu make double document add form is is checkbox enabled. but i cant find the way to take uploaded file id the is my object. and in image some logic explanation. maybe some one can tell what I am doing wrong? please.

<?php
class Document extends DataObject {
    private static $default_sort = 'SortOrder';
    private static $db = array(
        'SortOrder' => 'Int',
        'Title' => 'Varchar(255)',
        'HaveChild' => 'Boolean',
        'ParentDocumentID' => 'Int'
    );
    private static $has_one = array(
        'File' => 'File',
        'Category' => 'Category',
        'Location' => 'Location',
        'Product' => 'Product'
    );
    
    public function getCMSFields() {
        $fields = parent::getCMSFields();
        $fields = new FieldList(
            DropdownField::create('ProductID', 'Product', Product::get()->map('ID', 'Title'))->setEmptyString('(Select one)'),
            DropdownField::create('CategoryID', 'Category', Category::get()->map('ID', 'Title'))->setEmptyString('(Select one)')->displayIf("ProductID")->isNotEqualTo("")->end(),
            DropdownField::create('LocationID', 'Location', Location::get()->map('ID', 'Title'))->setEmptyString('(Select one)')->displayIf("ProductID")->isNotEqualTo("")->andIf("ProductID")->isNotEqualTo(3)->end(),
            TextField::create('Title', 'Title')->displayIf("ProductID")->isNotEqualTo("")->end(),
            UploadField::create('File', 'File')->setFolderName('Uploads/Files')->setAllowedExtensions(array('odt', 'jpg', 'jpeg', 'png', 'gif', 'doc', 'docx', 'xls', 'xlsx', 'ppt', 'pdf'))->displayIf("ProductID")->isNotEqualTo("")->end(),
                
            CheckboxField::create('HaveChild', 'I want to upload additional document')->displayIf("ProductID")->isEqualTo(5)->orIf("ProductID")->isEqualTo(6)->end(),
            DropdownField::create('CategoryID2', 'Category2', Category::get()->map('ID', 'Title'))->setEmptyString('(Select one)')->displayIf('HaveChild')->isChecked()->end(),
            DropdownField::create('LocationID2', 'Location2', Location::get()->map('ID', 'Title'))->setEmptyString('(Select one)')->displayIf('HaveChild')->isChecked()->end(),
            TextField::create('Title2', 'Title2')->displayIf('HaveChild')->isChecked()->end(),
            UploadField::create('File2', 'File2')->displayIf('HaveChild')->isChecked()->end()
        );
        
        
        return $fields;
    }
    
    private static $summary_fields = array(
        'SortOrder' => '#',
        'Location.Title' => 'Location',
        'Title' => 'Title',
        'Category.Title' => 'Category',
        'File.Name' => 'File',
        'File.Size' => 'Size',
        'LastEdited' => 'LastEdited',
        'Product.Name' => 'Product'
    );
    
    public function LocationDocumentTitle() {
        $lt = $this->Location()->Title;
        if ($lt == "" OR $lt == NULL)
        {
            return $this->Title;
        }
        else
        {
            return $this->Location()->Title . ' - ' . $this->Title;
        }
        
    }
    
    public function getFileToDownload() {
        $documentID = $this->ID;
    }
    
    
    public function onAfterWrite() {
        // 1. if selected add additional document
        if ($this->HaveChild == 1) {
            
            print_r($this);
            $parentID = $this->ID;
            
            // 2. if is anny file with productID associated
            $countOfParent = Document::get()->where("`Document`.`ParentDocumentID` = ".$parentID."")->Count();
            if ($countOfParent == 0) { // if 0 means first save then creation of sub document prossed
                $associatedDocument = new Document();
                $associatedDocument->Title = $this->Title2;
                $associatedDocument->CategoryID = $this->CategoryID2;
                $associatedDocument->LocationID = $this->LocationID2;
                
                if ($this->ProductID == 5) {
                    $associatedDocument->ProductID = 2;
                }
                elseif ($this->ProductID == 6) {
                    $associatedDocument->ProductID = 1;
                }
                $associatedDocument->ParentDocumentID = $parentID;
                
                //print_r($_POST['File2[Files][]']);
                
                //$associatedDocument->FileID = $this->File2;
                
                $associatedDocument->write();
            }
        }
        // else do nothing
        
        
        parent::onAfterWrite();
    }
}

Founded myself just used POST to get hidden input, $_POST['File2']['Files'][0]