diff -u b/core/modules/migrate_drupal_ui/src/MigrateUpgradeRunBatch.php b/core/modules/migrate_drupal_ui/src/MigrateUpgradeRunBatch.php
--- b/core/modules/migrate_drupal_ui/src/MigrateUpgradeRunBatch.php
+++ b/core/modules/migrate_drupal_ui/src/MigrateUpgradeRunBatch.php
@@ -106,6 +106,7 @@
     $definition = \Drupal::service('plugin.manager.migration')->getDefinition($migration_id);
     $configuration = [];
 
+    // @todo Find a way to avoid this in https://www.drupal.org/node/2804611.
     if ($definition['destination']['plugin'] === 'entity:file') {
       // Make sure we have a single trailing slash.
       $configuration['source']['constants']['source_base_path'] = rtrim($config['source_base_path'], '/') . '/';