diff --git a/core/modules/system/src/Form/ThemeSettingsForm.php b/core/modules/system/src/Form/ThemeSettingsForm.php
index 978ca80..049f91a 100644
--- a/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -287,7 +287,7 @@ 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(
+        $element['#description'] = t('Enter a path to an image. Drupal will look for the file relative to the Drupal root or the public file system. Explicitly using <code>public://</code> before the path will only look in the public file system. Examples: <code>@local-file</code> or <code>@explicit-file</code>', array(
           '@implicit-public-file' => isset($friendly_path) ? $friendly_path : $default,
           '@explicit-file' => file_uri_scheme($original_path) !== FALSE ? $original_path : 'public://' . $default,
           '@local-file' => $local_file,
@@ -361,7 +361,7 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
 
     if ($this->moduleHandler->moduleExists('file')) {
       // Handle file uploads.
-      $validators = array('file_validate_is_image' => array());
+      $validators = array('file_validate_extensions' => array('bmp png gif jpg jpeg apng svg'));
 
       // Check for a new uploaded logo.
       $file = file_save_upload('logo_upload', $validators, FALSE, 0);
diff --git a/core/modules/system/src/Tests/System/ThemeTest.php b/core/modules/system/src/Tests/System/ThemeTest.php
index 557a05c..b67a375 100644
--- a/core/modules/system/src/Tests/System/ThemeTest.php
+++ b/core/modules/system/src/Tests/System/ThemeTest.php
@@ -119,9 +119,8 @@ function testThemeSettings() {
         $explicit_file = 'public://' . $input;
         $local_file = PublicStream::basePath() . '/' . $input;
       }
-      $this->assertEqual((string) $elements[0], $implicit_public_file);
+      $this->assertEqual((string) $elements[0], $local_file);
       $this->assertEqual((string) $elements[1], $explicit_file);
-      $this->assertEqual((string) $elements[2], $local_file);
 
       // Verify the actual 'src' attribute of the logo being output in a site
       // branding block.