diff --git a/core/lib/Drupal/Core/Image/Image.php b/core/lib/Drupal/Core/Image/Image.php
index cc62f4a..3d99b21 100644
--- a/core/lib/Drupal/Core/Image/Image.php
+++ b/core/lib/Drupal/Core/Image/Image.php
@@ -101,6 +101,14 @@ public function __construct($source, ImageToolkitInterface $toolkit) {
    */
   public function getExtension() {
     $this->processInfo();
+
+    // Extension may still be empty after calling processInfo(). This happens if
+    // the image doesn't exists yet on disk, but only in memory as image
+    // resource. Try to extact the extension from the image source path.
+    if (empty($this->extension)) {
+      $this->extension = pathinfo($this->getSource(), PATHINFO_EXTENSION);
+    }
+
     return $this->extension;
   }
 
diff --git a/core/modules/image/lib/Drupal/image/Tests/ImageEffectsTest.php b/core/modules/image/lib/Drupal/image/Tests/ImageEffectsTest.php
index f00ecf1..95b329f 100644
--- a/core/modules/image/lib/Drupal/image/Tests/ImageEffectsTest.php
+++ b/core/modules/image/lib/Drupal/image/Tests/ImageEffectsTest.php
@@ -191,7 +191,6 @@ public function testImageSave() {
     $this->assertTrue($image
       ->setResource($res)
       ->setWidth(20)
-      ->setExtension('png')
       ->setHeight(20)
       ->save());