diff -u b/core/modules/media/src/Plugin/media/Source/File.php b/core/modules/media/src/Plugin/media/Source/File.php
--- b/core/modules/media/src/Plugin/media/Source/File.php
+++ b/core/modules/media/src/Plugin/media/Source/File.php
@@ -94,11 +94,11 @@
     $mimetype = $file->getMimeType();
     $mimetype = explode('/', $mimetype);
 
-    $icon_names = array(
+    $icon_names = [
       $mimetype[0] . '--' . $mimetype[1],
       $mimetype[1],
       $mimetype[0],
-    );
+    ];
     foreach ($icon_names as $icon_name) {
       $thumbnail = $icon_base . '/' . $icon_name . '.png';
       if (is_file($thumbnail)) {
diff -u b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceTestBase.php b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceTestBase.php
--- b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceTestBase.php
+++ b/core/modules/media/tests/src/FunctionalJavascript/MediaSourceTestBase.php
@@ -2,7 +2,6 @@
 
 namespace Drupal\Tests\media\FunctionalJavascript;
 
-use Drupal\Core\Entity\Entity\EntityFormDisplay;
 use Drupal\field\Entity\FieldConfig;
 use Drupal\field\Entity\FieldStorageConfig;
 use Drupal\media\Entity\MediaType;
diff -u b/core/modules/media/tests/src/Kernel/MediaSourceFileTest.php b/core/modules/media/tests/src/Kernel/MediaSourceFileTest.php
--- b/core/modules/media/tests/src/Kernel/MediaSourceFileTest.php
+++ b/core/modules/media/tests/src/Kernel/MediaSourceFileTest.php
@@ -2,10 +2,6 @@
 
 namespace Drupal\Tests\media\Kernel;
 
-use Drupal\media\Entity\Media;
-use Drupal\file\Entity\File;
-use org\bovigo\vfs\vfsStream;
-
 /**
  * Tests the file media source.
  *
@@ -17,57 +13,18 @@
    * Tests the file extension constraint.
    */
   public function testFileExtensionConstraint() {
+    $mediaType = $this->createMediaType('file');
     // Create a random file that should fail.
-    $media = $this->generateMedia('test.patch');
+    $media = $this->generateMedia('test.patch', $mediaType);
     $result = $media->validate();
     $this->assertCount(1, $result);
     $this->assertEquals('field_media_file.0', $result->get(0)->getPropertyPath());
     $this->assertContains('Only files with the following extensions are allowed:', (string) $result->get(0)->getMessage());
 
     // Create a random file that should pass.
-    $media = $this->generateMedia('test.txt');
+    $media = $this->generateMedia('test.txt', $mediaType);
     $result = $media->validate();
-    $this->assertCount(1, $result);
-    $this->assertEquals('field_media_file.0', $result->get(0)->getPropertyPath());
-    $this->assertNotContains('Only files with the following extensions are allowed:', (string) $result->get(0)->getMessage());
-  }
-
-  /**
-   * Helper to generate media entity.
-   *
-   * @param string $filename
-   *   String filename with extension.
-   *
-   * @return \Drupal\media\Entity\Media
-   *   A media entity.
-   */
-  protected function generateMedia($filename) {
-    $media_type = $this->createMediaType('file');
-
-    vfsStream::setup('drupal_root');
-    vfsStream::create([
-      'sites' => [
-        'default' => [
-          'files' => [
-            'test.patch' => str_repeat('a', 3000),
-          ]
-        ]
-      ]
-    ]);
-
-    $file = File::create([
-      'uri' => 'vfs://drupal_root/sites/default/files/' . $filename,
-    ]);
-    $file->setPermanent();
-    $file->save();
-
-    return Media::create([
-      'bundle' => $media_type->id(),
-      'name' => 'Mr. Jones',
-      'field_media_file' => [
-        'target_id' => $file->id(),
-      ]
-    ]);
+    $this->assertCount(0, $result);
   }
 
 }
only in patch2:
unchanged:
--- a/core/modules/media/tests/src/Kernel/MediaKernelTestBase.php
+++ b/core/modules/media/tests/src/Kernel/MediaKernelTestBase.php
@@ -2,8 +2,11 @@
 
 namespace Drupal\Tests\media\Kernel;
 
+use Drupal\file\Entity\File;
 use Drupal\KernelTests\KernelTestBase;
+use Drupal\media\Entity\Media;
 use Drupal\media\Entity\MediaType;
+use org\bovigo\vfs\vfsStream;
 
 /**
  * Base class for Media kernel tests.
@@ -88,4 +91,43 @@ protected function createMediaType($media_source_name) {
     return $media_type;
   }
 
+  /**
+   * Helper to generate media entity.
+   *
+   * @param string $filename
+   *   String filename with extension.
+   * @param \Drupal\media\Entity\MediaType $media_type
+   *   The the media type.
+   *
+   * @return \Drupal\media\Entity\Media
+   *   A media entity.
+   */
+  protected function generateMedia($filename, MediaType $media_type) {
+
+    vfsStream::setup('drupal_root');
+    vfsStream::create([
+      'sites' => [
+        'default' => [
+          'files' => [
+            $filename => str_repeat('a', 3000),
+          ],
+        ],
+      ],
+    ]);
+
+    $file = File::create([
+      'uri' => 'vfs://drupal_root/sites/default/files/' . $filename,
+    ]);
+    $file->setPermanent();
+    $file->save();
+
+    return Media::create([
+      'bundle' => $media_type->id(),
+      'name' => 'Mr. Jones',
+      'field_media_file' => [
+        'target_id' => $file->id(),
+      ],
+    ]);
+  }
+
 }