11 files declare their use of FileSecurity

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.

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