diff -u b/core/modules/system/src/Tests/File/StreamWrapperTest.php b/core/modules/system/src/Tests/File/StreamWrapperTest.php
--- b/core/modules/system/src/Tests/File/StreamWrapperTest.php
+++ b/core/modules/system/src/Tests/File/StreamWrapperTest.php
@@ -62,7 +62,7 @@
   }
 
   /**
-   * Test the file_stream_wrapper_get_instance_by_scheme() function.
+   * Test the getViaScheme() function.
    */
   function testGetInstanceByScheme() {
     $instance = \Drupal::service('stream_wrapper_manager')->getViaScheme($this->scheme);
only in patch2:
unchanged:
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -756,7 +756,7 @@ function file_save_upload($form_field_name, $validators = array(), $destination
       'uri' => $file_info->getRealPath(),
       'filesize' => $file_info->getSize(),
     );
-    $values['filemime'] = file_get_mimetype($values['filename']);
+    $values['filemime'] = \Drupal::service('file.mime_type.guesser')->guess($values['filename']);
     $file = entity_create('file', $values);
 
     $extensions = '';
only in patch2:
unchanged:
--- a/core/modules/file/src/Entity/File.php
+++ b/core/modules/file/src/Entity/File.php
@@ -185,7 +185,7 @@ public static function preCreate(EntityStorageInterface $storage, array &$values
 
     // Automatically detect filemime if not set.
     if (!isset($values['filemime']) && isset($values['uri'])) {
-      $values['filemime'] = file_get_mimetype($values['uri']);
+      $values['filemime'] = \Drupal::service('file.mime_type.guesser')->guess(($values['uri']);
     }
   }
 
only in patch2:
unchanged:
--- a/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -414,7 +414,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       }
 
       if (empty($values['default_favicon']) && !empty($values['favicon_path'])) {
-        $values['favicon_mimetype'] = file_get_mimetype($values['favicon_path']);
+        $values['favicon_mimetype'] = \Drupal::service('file.mime_type.guesser')->guess($values['favicon_path']);
       }
     }