alter-field-widget-in-forms.php

<?php

/**
 * Implements hook_field_widget_multivalue_form_alter().
 */
function MODULE_NAME_field_widget_multivalue_form_alter(&$elements, FormStateInterface $form_state, $context) {
  $field = $context['items']->getName();
  $build_info = $form_state->getBuildInfo();

  if ($field == "FIELD_NAME") {
    if (array_key_exists('form_id', $form_state->getBuildInfo())) {
      if ($build_info['form_id'] == 'node_CT_NAME_form' ||
        $build_info['form_id'] == 'node_CT_NAME_edit_form') {
        $i = 0;
        do {
          $elements[$key]['#process'][] = 'MODULE_NAME_process_FIELD_NAME';
        } while (isset($elements[++$i]));
      }
    }
  }
}

/**
 * The #process callback for alter field.
 *
 * Defined in hook_field_widget_multivalue_form_alter().
 */
function MODULE_NAME_process_FIELD_NAME($element, FormStateInterface $form_state) {
  // Change the field with widget element, you can alter imagen alt, imagen title, file descriptions, etc.

  return $element;
}