Fix ImageFieldMapping.

src/FieldMapping/ImageFieldMapping.php

@@ -26,6 +26,8 @@ class ImageFieldMapping extends FieldMapping {
     if (!in_array($extension, ['jpg', 'png', 'gif'])) {
       throw new \InvalidArgumentException('Extension must be one of: jpg, png, gif.');
     }
+
+    $this->extension = $extension;
   }
 
   /**

tests/src/Unit/FieldMapping/ImageFieldMappingTest.php

@@ -20,4 +20,9 @@ class ImageFieldMappingTest extends UnitTestCase {
     new ImageFieldMapping('Dummy image', 'bmp');
   }
 
+  public function testGetExtension() {
+    $image_field_mapping = new ImageFieldMapping('Dummy image', 'jpg');
+    self::assertEquals('jpg', $image_field_mapping->getExtension());
+  }
+
 }