diff --git a/core/modules/system/src/Form/ThemeSettingsForm.php b/core/modules/system/src/Form/ThemeSettingsForm.php
index 7f02517..5bece4c 100644
--- a/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -284,10 +284,9 @@ public function buildForm(array $form, FormStateInterface $form_state, $theme =
           $local_file = \Drupal::theme()->getActiveTheme()->getPath() . '/' . $default;
         }
 
-        $element['#description'] = t('Examples: <code>@implicit-public-file</code> (for a file in the public filesystem), <code>@explicit-file</code>, or <code>@local-file</code>.', array(
-          '@implicit-public-file' => isset($friendly_path) ? $friendly_path : $default,
-          '@explicit-file' => file_uri_scheme($original_path) !== FALSE ? $original_path : 'public://' . $default,
+        $element['#description'] = t('Enter a path to an image. Drupal will look for the file relative to the Drupal root or the public file system. Explicitly using <code>public://</code> before the path will only look in the public file system. Examples: <code>@local-file</code> or <code>@explicit-file</code>', array(
           '@local-file' => $local_file,
+          '@explicit-file' => file_uri_scheme($original_path) !== FALSE ? $original_path : 'public://' . $default,
         ));
       }
     }
@@ -358,7 +357,10 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
 
     if ($this->moduleHandler->moduleExists('file')) {
       // Handle file uploads.
-      $validators = array('file_validate_is_image' => array());
+      $image_factory = \Drupal::service('image.factory');
+      $supported_extensions = $image_factory->getSupportedExtensions();
+      $supported_extensions[] = 'svg';
+      $validators = ['file_validate_extensions' => [implode(' ', $supported_extensions)]];
 
       // Check for a new uploaded logo.
       $file = file_save_upload('logo_upload', $validators, FALSE, 0);
@@ -374,8 +376,6 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
         }
       }
 
-      $validators = array('file_validate_extensions' => array('ico png gif jpg jpeg apng svg'));
-
       // Check for a new uploaded favicon.
       $file = file_save_upload('favicon_upload', $validators, FALSE, 0);
       if (isset($file)) {
diff --git a/core/modules/system/src/Tests/System/ThemeTest.php b/core/modules/system/src/Tests/System/ThemeTest.php
index 278353d..ee9d12d 100644
--- a/core/modules/system/src/Tests/System/ThemeTest.php
+++ b/core/modules/system/src/Tests/System/ThemeTest.php
@@ -103,12 +103,10 @@ function testThemeSettings() {
         ':description' => 'description',
       ));
       // Expected default values (if all else fails).
-      $implicit_public_file = 'logo.svg';
       $explicit_file = 'public://logo.svg';
       $local_file = $default_theme_path . '/logo.svg';
       // Adjust for fully qualified stream wrapper URI in public filesystem.
       if (file_uri_scheme($input) == 'public') {
-        $implicit_public_file = file_uri_target($input);
         $explicit_file = $input;
         $local_file = strtr($input, array('public:/' => PublicStream::basePath()));
       }
@@ -118,13 +116,11 @@ function testThemeSettings() {
       }
       // Adjust for relative path within public filesystem.
       elseif ($input == file_uri_target($file->uri)) {
-        $implicit_public_file = $input;
         $explicit_file = 'public://' . $input;
         $local_file = PublicStream::basePath() . '/' . $input;
       }
-      $this->assertEqual((string) $elements[0], $implicit_public_file);
-      $this->assertEqual((string) $elements[1], $explicit_file);
-      $this->assertEqual((string) $elements[2], $local_file);
+      $this->assertEqual((string) $elements[1], $local_file);
+      $this->assertEqual((string) $elements[2], $explicit_file);
 
       // Verify the actual 'src' attribute of the logo being output in a site
       // branding block.