$form['actions']['submit']['#submit'][] = 'mymodule_upload_enabled_types_submit';

$form['actions']['submit']['#submit'][] = 'mymodule_what_ever_function';

$form['#submit'][1] = test_function;

unset($form["actions"]['submit']);

$form['actions']['submit']['#submit'][] = 'mymodule_upload_enabled_types_submit';

$form['#submit'][] = 'mymodule_upload_enabled_types_submit';

<?php

use DrupalCoreFormFormStateInterface;

/**
 * Implements hook_form_alter().
 */
function my_module_form_alter(&$form, FormStateInterface $form_state, $form_id) {

  // Use this to reveal the form id.
  //drupal_set_message($form_id);

  // Use this with the devel module to inspect the button action(s).
  //kint($form['actions']);

  switch ($form_id) {

    case 'node_article_form':      // New article nodes.
    case 'node_article_edit_form': // Existing article nodes.

      // Attach our custom submit handler.
      $form['actions']['publish']['#submit'][] = 'my_module_node_article_form_submit';
      break;

  }

}

function my_module_node_article_form_submit($form, FormStateInterface $form_state) {
  drupal_set_message('Running custom submit handler...');
}

unpublish
preview
delete