--- /dev/null
+++ b/core/modules/image/tests/Drupal/image/Tests/ImageStyleTest.php
@@ -0,0 +1,114 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\image\Tests\ImageStyleTest.
+ */
+
+namespace Drupal\image\Tests;
+
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Tests\UnitTestCase;
+use \Drupal\image\Entity\ImageStyle;
+
+/**
+ * @coversDefaultClass \Drupal\image\Entity\ImageStyle
+ *
+ * @group Drupal
+ * @group Image
+ */
+class ImageStyleTest extends UnitTestCase {
+
+  /**
+   * The entity type used for testing.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $entityType;
+
+  /**
+   * The entity manager used for testing.
+   *
+   * @var \Drupal\Core\Entity\EntityManagerInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $entityManager;
+
+  /**
+   * The ID of the type of the entity under test.
+   *
+   * @var string
+   */
+  protected $entityTypeId;
+
+  /**
+   * The effect manager used for testing.
+   *
+   * @var \Drupal\image\ImageEffectManager|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $effectManager;
+
+  /**
+   * The image effect used for testing.
+   *
+   * @var \Drupal\image\ImageEffectInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $imageEffect;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getInfo() {
+    return array(
+      'description' => '',
+      'name' => '\Drupal\image\Entity\ImageStyle unit test',
+      'group' => 'Image',
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    $this->entityTypeId = $this->randomName();
+    $this->provider = $this->randomName();
+    $this->entityType = $this->getMock('\Drupal\Core\Entity\EntityTypeInterface');
+    $this->entityType->expects($this->any())
+                     ->method('getProvider')
+                     ->will($this->returnValue($this->provider));
+    $this->entityManager = $this->getMock('\Drupal\Core\Entity\EntityManagerInterface');
+    $this->entityManager->expects($this->any())
+                        ->method('getDefinition')
+                        ->with($this->entityTypeId)
+                        ->will($this->returnValue($this->entityType));
+    $this->effectManager = $this->getMockBuilder('\Drupal\image\ImageEffectManager')
+        ->disableOriginalConstructor()
+        ->getMock();
+    $container = new ContainerBuilder();
+    \Drupal::setContainer($container);
+  }
+
+  /**
+   * @covers ::transformMimeType
+   */
+  public function testTransformMimeType() {
+    $image_effect_id = $this->randomName();
+    $image_effect = $this->getMockBuilder('\Drupal\image\ImageEffectBase')
+        ->setConstructorArgs(array(array(), $image_effect_id, array()))
+        ->getMock();
+    $image_effect->expects($this->any())
+        ->method('transformMimeType')
+        ->will($this->returnCallback(function (&$mime_type) { $mime_type = 'image/webp';}));
+    $this->effectManager->expects($this->any())
+        ->method('createInstance')
+        ->with($image_effect_id)
+        ->will($this->returnValue($image_effect));
+    \Drupal::getContainer()->set('plugin.manager.image.effect', $this->effectManager);
+
+    $image_style = new ImageStyle(array('effects' => array($image_effect_id => array('id' => $image_effect_id))), $this->entityTypeId);
+    $mime_types = array('image/jpeg', 'image/gif', 'image/png');
+    foreach ($mime_types as $mime_type) {
+      $image_style->transformMimeType($mime_type);
+      $this->assertEquals($mime_type, 'image/webp');
+    }
+  }
+}