How to use #element_validate and #value_callback in Drupal 8
To use an instance method for #element_validate/#value_callback etc you need a callable. e.g.

$form['upload'] = array(
  '#title' =&gt; t('Upload Images'),
  '#type' =&gt; 'managed_file',
  '#upload_location' =&gt; 'public://image_example_images/',
  '#element_validate' =&gt; array(
    array($this, 'file_generic_settings_max_filesize'),
  ),
);


You'll need to mark file_generic_settings_max_filesize() (which if you were being a stickler for standards should really be named fileGenericSettingsMaxFilesize) as public to accommodate, as your class won't be invoking the method.

Documentation for this is ongoing, the change notice is here.