Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/modules/file/config/schema/file.destination.schema.yml

@@ -0,0 +1,7 @@
+migrate.destination.entity:file:
+  type: migrate_destination
+  label: 'File'
+  mapping:
+    source_path_property:
+      type: string
+      label: 'Source path'

core/modules/file/config/schema/file.schema.yml

@@ -121,3 +121,11 @@ field.formatter.settings.file_extension:
     extension_detect_tar:
       type: boolean
       label: 'Detect tar'
+
+migrate.source.d6_upload_instance:
+  type: migrate_source_sql
+  label: 'Drupal 6 upload form display'
+  mapping:
+    constants:
+      type: migrate_entity_constant
+      label: 'Constants'