12 files declare their use of FileTestHelper

DeleteTest.php in core/modules/file/tests/src/Kernel/DeleteTest.php
DownloadTest.php in core/modules/file/tests/src/Functional/DownloadTest.php
FileManagedTestBase.php in core/modules/file/tests/src/Functional/FileManagedTestBase.php
FileManagedUnitTestBase.php in core/modules/file/tests/src/Kernel/FileManagedUnitTestBase.php
FileTestHooks.php in core/modules/file/tests/file_test/src/Hook/FileTestHooks.php
FileValidationTestSubscriber.php in core/modules/file/tests/modules/file_validator_test/src/EventSubscriber/FileValidationTestSubscriber.php
FileValidatorTest.php in core/modules/file/tests/src/Kernel/Validation/FileValidatorTest.php
LoadTest.php in core/modules/file/tests/src/Kernel/LoadTest.php
SaveTest.php in core/modules/file/tests/src/Kernel/SaveTest.php
SaveUploadFormTest.php in core/modules/file/tests/src/Functional/SaveUploadFormTest.php
SaveUploadTest.php in core/modules/file/tests/src/Functional/SaveUploadTest.php
ScanDirectoryTest.php in core/tests/Drupal/KernelTests/Core/File/ScanDirectoryTest.php

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