$_File array empty in Magento 2 while Uploading file at admin form

I am working on admin form in which i am trying to upload the file but it is giving me the $_File array is empty which means i am not getting file. Following is my code in form.

  $fieldset->addField(
        'file',
        'file',
         array( 
                'name' => 'file',
                'label' => __('File'),
                'title' => __('File')
            )
        );

Here is code of my controller

public function __construct(
        Context $context,        
        \Magento\Framework\Filesystem $filesystem,
        \Magento\MediaStorage\Model\File\UploaderFactory $fileUploaderFactory
    ) {
        $this->_mediaDirectory = $filesystem->getDirectoryWrite(\Magento\Framework\App\Filesystem\DirectoryList::MEDIA);
        $this->_fileUploaderFactory = $fileUploaderFactory;
        parent::__construct($context);
    }
    public function execute()
    {
        /** @var \Magento\Framework\Controller\Result\Redirect $resultRedirect */
        $resultRedirect = $this->resultRedirectFactory->create();
        try{
            $target = $this->_mediaDirectory->getAbsolutePath('mycustomfolder/');        
            /** @var $uploader \Magento\MediaStorage\Model\File\Uploader */
            $uploader = $this->_fileUploaderFactory->create(['fileId' => 'file']);
            /** Allowed extension types */
            $uploader->setAllowedExtensions(['jpg', 'jpeg', 'gif', 'png', 'zip', 'doc']);
            /** rename file name if already exists */
            $uploader->setAllowRenameFiles(true);
            /** upload file in folder "mycustomfolder" */
            $result = $uploader->save($target);
            if ($result['file']) {
                $this->messageManager->addSuccess(__('File has been successfully uploaded')); 
            }
        } catch (\Exception $e) {
            $this->messageManager->addError($e->getMessage());
        }
        return $this->resultRedirectFactory->create()->setPath(
            '*/*/upload', ['_secure'=>$this->getRequest()->isSecure()]
        );            
    }