How to create an 'Upload a file' field?

Question

I have a content entity type, and I would like to have an 'Upload file' field. I have declared the field in my Entity class

$fields['file'] = BaseFieldDefinition::create('file')
    ->setLabel(t('Upload a file'))
    ->setDescription(t('Choose file from your computer'));

And in EntityForm I am trying to set some settings, because I need to make this dependable later with the '#states'

$form['file'] = [
    '#title' => $this->t('File'),
    '#description' => $this->t('Upload a file from your computer'), 
    '#type' => 'file',
    '#default_value' => $entity->file->value,
];

But at the moment the field is displayed, but I have problems on saving the form. The file is attached, but not getting uploaded and the error says:

Fatal error: Call to a member function getValue() on a non-object in C:\A plus\drupalsite\core\modules\file\src\Plugin\Validation\Constraint\FileValidationConstraintValidator.php on line 18

Answer 1

 $form['File'] = array(
        '#type' => 'managed_file',
        '#title' => t('Choose  File'),
        '#upload_location' => 'public://MyFilesFolder/',
        '#default_value' => $entity->get('File')->value,
        '#description' => t('upload file'),
        '#states' => array(
          'visible' => array(
            ':input[name="File_type"]' => array('value' => t('Upload Your File')),
          ),
        ),
      );