Issue #1839068 by Berdir, fago: Implement the new entity field API for the file field type.

core/modules/field/field.module

core/modules/file/file.field.inc

core/modules/file/file.module

...upal/file/Plugin/field/formatter/GenericFileFormatter.php

...dules/file/lib/Drupal/file/Tests/FileFieldDisplayTest.php

...odules/file/lib/Drupal/file/Tests/FileFieldWidgetTest.php

core/modules/file/lib/Drupal/file/Tests/FileItemTest.php

core/modules/file/lib/Drupal/file/Type/FileItem.php