diff -u b/core/modules/migrate_drupal_ui/src/Batch/MigrateUpgradeImportBatch.php b/core/modules/migrate_drupal_ui/src/Batch/MigrateUpgradeImportBatch.php
--- b/core/modules/migrate_drupal_ui/src/Batch/MigrateUpgradeImportBatch.php
+++ b/core/modules/migrate_drupal_ui/src/Batch/MigrateUpgradeImportBatch.php
@@ -111,8 +111,8 @@
     $definition = \Drupal::service('plugin.manager.migration')->getDefinition($migration_id);
     $configuration = [];
 
-    // Set the source plugin constant, source_base_path, for all migrations
-    // with a file entity destination.
+    // Set the source plugin constant, source_base_path, for all migrations with
+    // a file entity destination.
     // @todo https://www.drupal.org/node/2804611.
     //   Find a way to avoid having to set configuration here.
     if ($definition['destination']['plugin'] === 'entity:file') {
diff -u b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeExecuteTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeExecuteTestBase.php
--- b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeExecuteTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeExecuteTestBase.php
@@ -172,7 +172,7 @@
   /**
    * Provides the source base path for private files for the credential form.
    *
-   * @return string
+   * @return string|null
    *   The source base path.
    */
   protected function getSourcePrivateBasePath() {
reverted:
--- b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/IdConflictTest.php
+++ a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/IdConflictTest.php
@@ -43,12 +43,6 @@
     return __DIR__ . '/files';
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  protected function getSourcePrivateBasePath() {
-  }
-
   /**
    * {@inheritdoc}
    */
reverted:
--- b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/NodeClassicTest.php
+++ a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/NodeClassicTest.php
@@ -50,12 +50,6 @@
     return __DIR__ . '/files';
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  protected function getSourcePrivateBasePath() {
-  }
-
   /**
    * {@inheritdoc}
    */
reverted:
--- b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/Upgrade6Test.php
+++ a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/Upgrade6Test.php
@@ -65,12 +65,6 @@
     return __DIR__ . '/files';
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  protected function getSourcePrivateBasePath() {
-  }
-
   /**
    * {@inheritdoc}
    */
diff -u b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/FilePathTest.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/FilePathTest.php
--- b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/FilePathTest.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/FilePathTest.php
@@ -28,21 +28,24 @@
   protected $defaultTheme = 'stark';
 
   /**
-   * The file system.
+   * The file system service.
    *
-   * @var \Drupal\Core\File\FileSystem
+   * @var \Drupal\Core\File\FileSystemInterface
    */
   protected $fs;
 
   /**
-   * The file path variables in the source database.
+   * The base path to the source files on the destination site.
    *
    * @var string[]
    */
   protected $basePath = [];
 
   /**
-   * The file path variables in the source database.
+   * The file scheme variables in the source database.
+   *
+   * These are 'file_private_path', 'file_public_path', and
+   * 'file_temporary_path',
    *
    * @var string[]
    */
@@ -83,8 +86,6 @@
    * @param string $temporary
    *   The path to the source temporary files.
    *
-   * @throws \Drupal\migrate\MigrateException
-   *
    * @dataProvider providerTestFilePath
    */
   public function testFilePath($file_private_path, $file_public_path, $file_temporary_path, $private, $public, $temporary) {
@@ -140,13 +141,11 @@
     $this->drupalPostForm(NULL, $edits, 'Review upgrade');
 
     // The migrations are now in store - remove all but the file migrations.
-    $tempstore_private = \Drupal::service('tempstore.private');
-    $store = $tempstore_private->get('migrate_drupal_ui');
-    $all_migrations = $store->get('migrations');
-    $migration_array = array_intersect_key($all_migrations, [
-      'd7_file' => [],
-      'd7_file_private' => [],
-    ]);
+    $store = \Drupal::service('tempstore.private')->get('migrate_drupal_ui');
+    $migration_array = array_intersect_key(
+      $store->get('migrations'),
+      array_flip(['d7_file', 'd7_file_private'])
+    );
     $store->set('migrations', $migration_array);
 
     // Perform the migrations.
@@ -163,41 +162,37 @@
    */
   public function providerTestFilePath() {
     return [
-      // All source base paths are at temporary://.
-      [
-        'file_private_path' => 'sites/default/private',
-        'file_public_path' => 'sites/default/files',
-        'file_temporary_path' => '/tmp',
-        'private' => '',
-        'public' => '',
-        'temporary' => '',
+      'All source base paths are at temporary' => [
+        'sites/default/private',
+        'sites/default/files',
+        '/tmp',
+        '',
+        '',
+        '',
       ],
-      // The private files are in a subdirectory.
-      [
-        'file_private_path' => 'sites/default/private',
-        'file_public_path' => 'sites/default/files',
-        'file_temporary_path' => '/tmp',
-        'private' => 'abc',
-        'public' => '',
-        'temporary' => '',
+      'The private files are in a subdirectory' => [
+        'sites/default/private',
+        'sites/default/files',
+        '/tmp',
+        'abc',
+        '',
+        '',
       ],
-      // The public files are in a subdirectory.
-      [
-        'file_private_path' => 'sites/default/private',
-        'file_public_path' => 'sites/default/files',
-        'file_temporary_path' => '/tmp',
-        'private' => '',
-        'public' => 'def',
-        'temporary' => '',
+      ' The public files are in a subdirectory' => [
+        'sites/default/private',
+        'sites/default/files',
+        '/tmp',
+        '',
+        'def',
+        '',
       ],
-      // The private and public files are in separate subdirectories.
-      [
-        'file_private_path' => 'private',
-        'file_public_path' => 'files',
-        'file_temporary_path' => '/tmp',
-        'private' => 'abc',
-        'public' => 'def',
-        'temporary' => 'xyz',
+      'The private and public files are in separate subdirectories' => [
+        'private',
+        'files',
+        '/tmp',
+        'abc',
+        'def',
+        'xyz',
       ],
     ];
   }
@@ -213,11 +208,16 @@
     // Get file information from the source database.
     $files = $this->getManagedFiles();
     foreach ($files as $file) {
-      preg_match('/^(private|public|temporary):/', $file['uri'], $matches);
+      $this->assertSame(1, preg_match('/^(private|public|temporary):/', $file['uri'], $matches));
+      //assertSame(1, preg_match('/^(private|public|temporary):/', $file['uri'], $matches));
       $scheme = $matches[1];
-      $path = $this->variable[$scheme] ?: '';
-      $base_path = $this->getSourcePath($scheme) . '/' . $path;
-      $filepath = $base_path . '/' . $file['filename'];
+      $path = $this->variable[$scheme] ?? '';
+      $filepath = implode('/', [
+        $this->getSourcePath($scheme),
+        $path,
+        $file['filename'],
+      ]);
+
       // Create the file.
       $source_file = @fopen($filepath, 'w');
       if (!$source_file) {
reverted:
--- b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/IdConflictTest.php
+++ a/core/modules/migrate_drupal_ui/tests/src/Functional/d7/IdConflictTest.php
@@ -45,12 +45,6 @@
     return __DIR__ . '/files';
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  protected function getSourcePrivateBasePath() {
-  }
-
   /**
    * {@inheritdoc}
    */