How to place all form buttons on top of the page, before the form, in all forms? Since these forms can get quite big sometimes, especially with a lot of files attached and default preview for them.

Related, but not the same: How can I display a form button on top of the form?
You can do that using hook_form_alter() and setting the weight of $form['actions'] to a very small negative number.  Note that this applies only those buttons wrapped in $form['actions']. For other buttons that they aren't, you should add them manually.

function YOURMODULE_form_alter(&amp;$form, &amp;$form_state) {
  $form['actions']['#weight'] = -1000;
  // a button that is not wrapped in actions.
  // assuming it's id is 'submit'
  $form['submit']['#weight'] = -1000;
}