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
@@ -4,6 +4,7 @@
 
 use Drupal\file\FileInterface;
 use Drupal\media\MediaInterface;
+use Drupal\media\MediaTypeInterface;
 use Drupal\media\MediaSourceBase;
 
 /**
@@ -110,2 +111,20 @@
 
+  /**
+   * {@inheritdoc}
+   */
+  public function createSourceField(MediaTypeInterface $type) {
+    $storage = $this->getSourceFieldStorage() ?: $this->createSourceFieldStorage();
+    return $this->entityTypeManager
+      ->getStorage('field_config')
+      ->create([
+        'field_storage' => $storage,
+        'bundle' => $type->id(),
+        'label' => $this->pluginDefinition['label'],
+        'required' => TRUE,
+        'settings' => [
+          'file_extensions' => 'txt doc docx pdf'
+        ]
+      ]);
+  }
+
 }
diff -u b/core/profiles/standard/config/optional/field.field.media.file.field_media_file.yml b/core/profiles/standard/config/optional/field.field.media.file.field_media_file.yml
--- b/core/profiles/standard/config/optional/field.field.media.file.field_media_file.yml
+++ b/core/profiles/standard/config/optional/field.field.media.file.field_media_file.yml
@@ -21,7 +21,7 @@
 default_value_callback: ''
 settings:
   file_directory: '[date:custom:Y]-[date:custom:m]'
-  file_extensions: 'txt'
+  file_extensions: 'txt doc docx pdf'
   max_filesize: ''
   handler: 'default:file'
   handler_settings: {  }
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/media/tests/src/Kernel/MediaSourceFileTest.php
@@ -0,0 +1,52 @@
+<?php
+
+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.
+ *
+ * @group media
+ */
+class MediaSourceFileTest extends MediaKernelTestBase {
+  /**
+   * Tests the file extension constraint.
+   */
+  public function testFileExtensionConstraint() {
+    $media_type = $this->createMediaType('file');
+
+    vfsStream::setup('drupal_root');
+    vfsStream::create([
+      'sites' => [
+        'default' => [
+          'files' => [
+            'test.patch' => str_repeat('a', 3000),
+          ]
+        ]
+      ]
+    ]);
+
+    // Create a random file that should fail.
+    $file = File::create([
+      'uri' => 'vfs://drupal_root/sites/default/files/test.patch',
+    ]);
+    $file->setPermanent();
+    $file->save();
+
+    $media = Media::create([
+      'bundle' => $media_type->id(),
+      'name' => 'Mr. Jones',
+      'field_media_file' => [
+        'target_id' => $file->id(),
+      ]
+    ]);
+
+    $result = $media->validate();
+    $this->assertCount(1, $result);
+    $this->assertEquals('field_media_file.0', $result->get(0)->getPropertyPath());
+    $this->assertEquals('Only files with the following extensions are allowed: <em class="placeholder">txt doc docx pdf</em>.', (string) $result->get(0)->getMessage());
+  }
+}
\ No newline at end of file