11 files declare their use of FileSecurity in Drupal 8

Composer.php in core/lib/Drupal/Core/Composer/Composer.php

DirectoryTest.php in core/tests/Drupal/KernelTests/Core/File/DirectoryTest.php

file.inc in core/includes/file.inc

API for handling file uploads and server file management.

FileSaveHtaccessLoggingTest.php in core/modules/system/tests/src/Functional/File/FileSaveHtaccessLoggingTest.php

FileSecurityTest.php in core/tests/Drupal/Tests/Component/FileSecurity/FileSecurityTest.php

FileStorage.php in core/lib/Drupal/Core/Config/FileStorage.php

FileStorage.php in core/lib/Drupal/Component/PhpStorage/FileStorage.php

HtaccessWriter.php in core/lib/Drupal/Core/File/HtaccessWriter.php

MTimeProtectedFileStorageBase.php in core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageBase.php

simpletest.install in core/modules/simpletest/simpletest.install

Install, update and uninstall functions for the simpletest module.

system.module in core/modules/system/system.module

Configuration system that lets administrators modify the workings of the site.