Issue #2066219 by claudiu.cristea, fietserwin: Decouple image type from image extension.

core/lib/Drupal/Core/Image/Image.php

core/lib/Drupal/Core/Image/ImageInterface.php

core/modules/file/file.module

core/modules/image/image.field.inc

core/modules/image/image.module

...ge/lib/Drupal/image/Plugin/field/field_type/ImageItem.php

...ystem/lib/Drupal/system/Plugin/ImageToolkit/GDToolkit.php

...system/lib/Drupal/system/Plugin/ImageToolkitInterface.php

...es/system/lib/Drupal/system/Tests/Image/ToolkitGdTest.php

...lib/Drupal/image_test/Plugin/ImageToolkit/TestToolkit.php

core/tests/Drupal/Tests/Core/Image/ImageTest.php