Update to Drupal 8.1.5. For more information, see https://www.drupal.org/project/drupal/releases/8.1.5

core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatter.php

@@ -188,14 +188,14 @@ class ImageFormatter extends ImageFormatterBase implements ContainerFactoryPlugi
     $image_style_setting = $this->getSetting('image_style');
 
     // Collect cache tags to be added for each item in the field.
-    $cache_tags = array();
+    $base_cache_tags = [];
     if (!empty($image_style_setting)) {
       $image_style = $this->imageStyleStorage->load($image_style_setting);
-      $cache_tags = $image_style->getCacheTags();
+      $base_cache_tags = $image_style->getCacheTags();
     }
 
     foreach ($files as $delta => $file) {
-      $cache_contexts = array();
+      $cache_contexts = [];
       if (isset($link_file)) {
         $image_uri = $file->getFileUri();
         // @todo Wrap in file_url_transform_relative(). This is currently
@@ -206,7 +206,7 @@ class ImageFormatter extends ImageFormatterBase implements ContainerFactoryPlugi
         $url = Url::fromUri(file_create_url($image_uri));
         $cache_contexts[] = 'url.site';
       }
-      $cache_tags = Cache::mergeTags($cache_tags, $file->getCacheTags());
+      $cache_tags = Cache::mergeTags($base_cache_tags, $file->getCacheTags());
 
       // Extract field item attributes for the theme function, and unset them
       // from the $item so that the field template does not re-render them.

core/modules/image/src/Plugin/Field/FieldType/ImageItem.php

@@ -343,7 +343,7 @@ class ImageItem extends FileItem {
         $image = File::create();
         $image->setFileUri($path);
         $image->setOwnerId(\Drupal::currentUser()->id());
-        $image->setMimeType('image/' . pathinfo($path, PATHINFO_EXTENSION));
+        $image->setMimeType(\Drupal::service('file.mime_type.guesser')->guess($path));
         $image->setFileName(drupal_basename($path));
         $destination_dir = static::doGetUploadLocation($settings);
         file_prepare_directory($destination_dir, FILE_CREATE_DIRECTORY);

core/modules/image/tests/src/Kernel/ImageFormatterTest.php

@@ -0,0 +1,102 @@
+<?php
+
+namespace Drupal\Tests\image\Kernel;
+
+use Drupal\Component\Utility\Unicode;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\entity_test\Entity\EntityTest;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\Tests\field\Kernel\FieldKernelTestBase;
+
+/**
+ * Tests the image field rendering using entity fields of the image field type.
+ *
+ * @group image
+ */
+class ImageFormatterTest extends FieldKernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('file', 'image');
+
+  /**
+   * @var string
+   */
+  protected $entityType;
+
+  /**
+   * @var string
+   */
+  protected $bundle;
+
+  /**
+   * @var string
+   */
+  protected $fieldName;
+
+  /**
+   * @var \Drupal\Core\Entity\Display\EntityViewDisplayInterface
+   */
+  protected $display;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->installConfig(['field']);
+    $this->installEntitySchema('entity_test');
+    $this->installEntitySchema('file');
+    $this->installSchema('file', array('file_usage'));
+
+    $this->entityType = 'entity_test';
+    $this->bundle = $this->entityType;
+    $this->fieldName = Unicode::strtolower($this->randomMachineName());
+
+    FieldStorageConfig::create(array(
+      'entity_type' => $this->entityType,
+      'field_name' => $this->fieldName,
+      'type' => 'image',
+      'cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED,
+    ))->save();
+    FieldConfig::create([
+      'entity_type' => $this->entityType,
+      'field_name' => $this->fieldName,
+      'bundle' => $this->bundle,
+      'settings' => [
+        'file_extensions' => 'jpg',
+      ],
+    ])->save();
+
+    $this->display = entity_get_display($this->entityType, $this->bundle, 'default')
+      ->setComponent($this->fieldName, [
+        'type' => 'image',
+        'label' => 'hidden',
+      ]);
+    $this->display->save();
+  }
+
+  /**
+   * Tests the cache tags from image formatters.
+   */
+  function testImageFormatterCacheTags() {
+    // Create a test entity with the image field set.
+    $entity = EntityTest::create([
+      'name' => $this->randomMachineName(),
+    ]);
+    $entity->{$this->fieldName}->generateSampleItems(2);
+    $entity->save();
+
+    // Generate the render array to verify if the cache tags are as expected.
+    $build = $this->display->build($entity);
+
+    $this->assertEquals($entity->{$this->fieldName}[0]->entity->getCacheTags(), $build[$this->fieldName][0]['#cache']['tags'], 'First image cache tags is as expected');
+    $this->assertEquals($entity->{$this->fieldName}[1]->entity->getCacheTags(), $build[$this->fieldName][1]['#cache']['tags'], 'Second image cache tags is as expected');
+  }
+
+}

core/modules/image/tests/src/Kernel/ImageItemTest.php

@@ -53,6 +53,9 @@ class ImageItemTest extends FieldKernelTestBase {
       'entity_type' => 'entity_test',
       'field_name' => 'image_test',
       'bundle' => 'entity_test',
+      'settings' => [
+        'file_extensions' => 'jpg',
+      ],
     ])->save();
     file_unmanaged_copy(\Drupal::root() . '/core/misc/druplicon.png', 'public://example.jpg');
     $this->image = File::create([
@@ -123,6 +126,7 @@ class ImageItemTest extends FieldKernelTestBase {
     $entity = EntityTest::create();
     $entity->image_test->generateSampleItems();
     $this->entityValidateAndSave($entity);
+    $this->assertEqual($entity->image_test->entity->get('filemime')->value, 'image/jpeg');
   }
 
 }

core/modules/image/tests/src/Unit/ImageStyleTest.php

@@ -41,7 +41,7 @@ class ImageStyleTest extends UnitTestCase {
    * @param \Drupal\image\ImageEffectInterface|\PHPUnit_Framework_MockObject_MockObject $image_effect
    *   The image effect used for testing.
    *
-   * @return \Drupal\image\ImageStyleInterface|\Drupal\image\ImageStyleInterface
+   * @return \Drupal\image\ImageStyleInterface
    *   The mocked image style.
    */
   protected function getImageStyleMock($image_effect_id, $image_effect, $stubs = array()) {