diff -u b/core/modules/file/tests/src/Unit/Plugin/migrate/source/d7/FileTest.php b/core/modules/file/tests/src/Unit/Plugin/migrate/source/d7/FileTest.php
--- b/core/modules/file/tests/src/Unit/Plugin/migrate/source/d7/FileTest.php
+++ b/core/modules/file/tests/src/Unit/Plugin/migrate/source/d7/FileTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Tests\file\Unit\Plugin\migrate\source\d7;
 
+use Drupal\migrate\Row;
 use Drupal\Tests\migrate\Unit\MigrateSqlSourceTestCase;
 
 /**
@@ -50,2 +51,38 @@
 
+  protected function setProperty($property, $value) {
+    $reflector = (new \ReflectionClass($this->source))->getProperty($property);
+    $reflector->setAccessible(TRUE);
+    $reflector->setValue($this->source, $value);
+  }
+
+  /**
+   * Tests that public file URIs are properly transformed by prepareRow().
+   */
+  public function testPublicUri() {
+    $this->setProperty('publicPath', 'sites/default/files');
+    $row = new Row(['uri' => 'public://foo.png'], ['uri' => []]);
+    $this->source->prepareRow($row);
+    $this->assertEquals('sites/default/files/foo.png', $row->getSourceProperty('filepath'));
+  }
+
+  /**
+   * Tests that private file URIs are properly transformed by prepareRow().
+   */
+  public function testPrivateUri() {
+    $this->setProperty('privatePath', '/path/to/private/files');
+    $row = new Row(['uri' => 'private://baz.jpeg'], ['uri' => []]);
+    $this->source->prepareRow($row);
+    $this->assertEquals('/path/to/private/files/baz.jpeg', $row->getSourceProperty('filepath'));
+  }
+
+  /**
+   * Tests that temporary file URIs are property transformed by prepareRow().
+   */
+  public function testTemporaryUri() {
+    $this->setProperty('temporaryPath', '/tmp');
+    $row = new Row(['uri' => 'temporary://camelot/lancelot.gif'], ['uri' => []]);
+    $this->source->prepareRow($row);
+    $this->assertEquals('/tmp/camelot/lancelot.gif', $row->getSourceProperty('filepath'));
+  }
+
 }