Issue #2259567: Unnecessary restrictions on logo format: Can't upload replacement SVG logo

Rebased existing MR on 9.3.x

core/modules/system/src/Form/ThemeSettingsForm.php

@@ -236,7 +236,9 @@ public function buildForm(array $form, FormStateInterface $form_state, $theme =
         '#maxlength' => 40,
         '#description' => $this->t("If you don't have direct file access to the server, use this field to upload your logo."),
         '#upload_validators' => [
-          'file_validate_is_image' => [],
+          'file_validate_extensions' => [
+            'png gif jpg jpeg apng svg',
+          ],
         ],
       ];
     }