How to allow any file extension (or use a blacklist instead of a whitelist) for file fields in Drupal 8?

Question

For a file field, it’s required to enter file extensions in "Allowed file extensions".

Is there a way to allow any file extension? Or a way to switch from a whitelist to a blacklist?

What I found

• For Drupal 7, there is the module Allow All File Extensions. A Drupal 8 port doesn’t seem to be planned.

• For Drupal 7, there is the question How to allow for files of any extension to be uploaded?, where the module mentioned above is recommended.

• For Drupal 8, there is the issue Not possible to allow uploading files with any file extension, but it seems to be inactive.

Answer 1

Implement a hook alter on the node form or the field level and in it change the validator to your custom function:

/**
 * 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;

}

Alternatively you can change the behavior of the field on any node, or entity by altering the widget form:

function mymodule_field_widget_form_alter(&$element, &$form_state, $context) {  
   if ( "condition to check if it's your field" == TRUE) {
     $upload_validators = array(
            'my_module_file_validate_extensions_blacklist' => ['jpg jpeg gif png mp4']
        );
        $element['#upload_validators'] = $upload_validators;
  }
}

Then write you own function to do what you want:

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
}