diff -u b/core/modules/file/migration_templates/d6_file.yml b/core/modules/file/migration_templates/d6_file.yml
--- b/core/modules/file/migration_templates/d6_file.yml
+++ b/core/modules/file/migration_templates/d6_file.yml
@@ -11,8 +11,7 @@
   filename: filename
   source_full_path:
     -
-      plugin: concat
-      delimiter: /
+      plugin: d6_file_source
       source:
         - constants/source_base_path
         - filepath
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/file/src/Plugin/migrate/process/d6/FileSource.php
@@ -0,0 +1,32 @@
+<?php
+
+namespace Drupal\file\Plugin\migrate\process\d6;
+
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+
+/**
+ * Generates the source full path.
+ *
+ * @MigrateProcessPlugin(
+ *   id = "d6_file_source"
+ * )
+ */
+class FileSource extends ProcessPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    list($source_base_path, $filepath) = $value;
+    if (($source_base_path == '') || ($source_base_path[0] = '/')) {
+      $source = $filepath;
+    }
+    else {
+      $source = $source_base_path . '/' . $filepath;
+    }
+    return $source;
+  }
+
+}