Issue #3487761 by jastraat, terracoders: file_validate functions are deprecated, removed in D11

src/Form/AddSectionToLibraryForm.php

@@ -121,7 +121,7 @@ class AddSectionToLibraryForm extends FormBase {
       '#multiple' => FALSE,
       '#upload_location' => 'public://',
       '#upload_validators' => [
-        'file_validate_extensions' => ['gif png jpg jpeg'],
+        'FileExtension' => ['gif png jpg jpeg'],
       ],
     ];
 

src/Form/AddTemplateToLibraryForm.php

@@ -121,7 +121,7 @@ class AddTemplateToLibraryForm extends FormBase {
       '#multiple' => FALSE,
       '#upload_location' => 'public://',
       '#upload_validators' => [
-        'file_validate_extensions' => ['gif png jpg jpeg'],
+        'FileExtension' => ['gif png jpg jpeg'],
       ],
     ];