Issue #2869239 : Action to refresh the thumbnail

config/install/system.action.media_reset_thumbnail_action.yml

@@ -0,0 +1,10 @@
+langcode: en
+status: true
+dependencies:
+  module:
+    - media_entity
+id: media_reset_thumbnail_action
+label: 'Reset media thumbnail'
+type: media
+plugin: media_reset_thumbnail_action
+configuration: {  }

src/Plugin/Action/ResetMediaThumbnail.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace Drupal\media_entity\Plugin\Action;
+
+use Drupal\Core\Action\ActionBase;
+use Drupal\Core\Session\AccountInterface;
+
+/**
+ * Provides an action to reset the thumbnail on a media entity.
+ *
+ * @Action(
+ *   id = "media_reset_thumbnail_action",
+ *   label = @Translation("Reset media thumbnail"),
+ *   type = "media"
+ * )
+ */
+class ResetMediaThumbnail extends ActionBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function execute($entity = NULL) {
+    $entity->automaticallySetThumbnail();
+    // We need to change at least one value, otherwise the changed timestamp
+    // will not be updated.
+    $entity->changed = 0;
+    $entity->save();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function access($object, AccountInterface $account = NULL, $return_as_object = FALSE) {
+    /** @var \Drupal\media_entity\MediaInterface $object */
+    return $object->access('update', $account, $return_as_object);
+  }
+
+}