class FileTestForm

class FileTestForm implements FormInterface {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return '_file_test_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['file_test_upload'] = [
            '#type' => 'file',
            '#title' => t('Upload a file'),
        ];
        $form['file_test_replace'] = [
            '#type' => 'select',
            '#title' => t('Replace existing image'),
            '#options' => [
                FileSystemInterface::EXISTS_RENAME => t('Appends number until name is unique'),
                FileSystemInterface::EXISTS_REPLACE => t('Replace the existing file'),
                FileSystemInterface::EXISTS_ERROR => t('Fail with an error'),
            ],
            '#default_value' => FileSystemInterface::EXISTS_RENAME,
        ];
        $form['file_subdir'] = [
            '#type' => 'textfield',
            '#title' => t('Subdirectory for test file'),
            '#default_value' => '',
        ];
        $form['extensions'] = [
            '#type' => 'textfield',
            '#title' => t('Allowed extensions.'),
            '#default_value' => '',
        ];
        $form['allow_all_extensions'] = [
            '#title' => t('Allow all extensions?'),
            '#type' => 'radios',
            '#options' => [
                'false' => 'No',
                'empty_array' => 'Empty array',
                'empty_string' => 'Empty string',
            ],
            '#default_value' => 'false',
        ];
        $form['is_image_file'] = [
            '#type' => 'checkbox',
            '#title' => t('Is this an image file?'),
            '#default_value' => TRUE,
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // Process the upload and perform validation. Note: we're using the
        // form value for the $replace parameter.
        if (!$form_state->isValueEmpty('file_subdir')) {
            $destination = 'temporary://' . $form_state->getValue('file_subdir');
            \Drupal::service('file_system')->prepareDirectory($destination, FileSystemInterface::CREATE_DIRECTORY);
        }
        else {
            $destination = FALSE;
        }
        // Setup validators.
        $validators = [];
        if ($form_state->getValue('is_image_file')) {
            $validators['file_validate_is_image'] = [];
        }
        $allow = $form_state->getValue('allow_all_extensions');
        if ($allow === 'empty_array') {
            $validators['file_validate_extensions'] = [];
        }
        elseif ($allow === 'empty_string') {
            $validators['file_validate_extensions'] = [
                '',
            ];
        }
        elseif (!$form_state->isValueEmpty('extensions')) {
            $validators['file_validate_extensions'] = [
                $form_state->getValue('extensions'),
            ];
        }
        // The test for \Drupal::service('file_system')->moveUploadedFile()
        // triggering a warning is unavoidable. We're interested in what happens
        // afterwards in file_save_upload().
        if (\Drupal::state()->get('file_test.disable_error_collection')) {
            define('SIMPLETEST_COLLECT_ERRORS', FALSE);
        }
        $file = file_save_upload('file_test_upload', $validators, $destination, 0, $form_state->getValue('file_test_replace'));
        if ($file) {
            $form_state->setValue('file_test_upload', $file);
            \Drupal::messenger()->addStatus(t('File @filepath was uploaded.', [
                '@filepath' => $file->getFileUri(),
            ]));
            \Drupal::messenger()->addStatus(t('File name is @filename.', [
                '@filename' => $file->getFilename(),
            ]));
            \Drupal::messenger()->addStatus(t('File MIME type is @mimetype.', [
                '@mimetype' => $file->getMimeType(),
            ]));
            \Drupal::messenger()->addStatus(t('You WIN!'));
        }
        elseif ($file === FALSE) {
            \Drupal::messenger()->addError(t('Epic upload FAIL!'));
        }
    }

}