public function UploadAvatarForm()
{
    $upload_folder = self::getUserUploadFolder() . $folder_name;

    $image_field = FileField::create('Avatar', 'Avatar');
    $image_field->setFolderName($upload_folder);
    $image_field->getValidator()->setAllowedExtensions(array('jpg', 'jpeg', 'png'));
    $image_field->getValidator()->setAllowedMaxFileSize(1 * 1024 * 1024); // 1 MB in bytes

    $fields = FieldList::create(
        $image_field
    );

    $actions = FieldList::create(
        FormAction::create('submitUploadAvatarForm', 'Upload Avatar')->addExtraClass('js-upload-image-form-submit')
    );

    $validator = RequiredFields::create();

    $form = Form::create($this, 'UploadAvatarForm', $fields, $actions, $validator)->addExtraClass('upload-image-form js-upload-image-form');

    return $form;
}

public function submitUploadAvatarForm($data, $form)
{
    $current_user = User::currentUser();
    $form->saveInto($current_user);
    $current_user->write();

    return 'success';
}

class User extends DataObject
{
    private static $has_one = array(
        'Avatar' => 'Image'
    );
}