/**
 * alter form field element:.
 *
 * Implements hook_FORM_alter
 */
function my_module_form_alter(&$form, &$form_state, $form_id) {
    $upload_validators = array(
        'my_module_file_validate_extensions_blacklist' => ['jpg jpeg gif png mp4']
    );
    $form['your_file_field_element']['#upload_validators'] = $upload_validators;

}

function my_module_file_validate_extensions_blacklist(stdClass $file, $extensions) {
  $errors = array();
  $regex = '/\.(' . preg_replace('/ +/', '|', preg_quote($extensions)) . ')$/i';
  if (preg_match($regex, $file->filename)) {
    $errors[] = t('files with the following extensions are not allowed: %files-allowed.', array(
      '%files-allowed' => $extensions,
    ));
  }
  return $errors
}