10 files declare their use of FileExistsException

CopyTest.php in core/modules/file/tests/src/Kernel/CopyTest.php
file.module in core/modules/file/file.module
Defines a "managed_file" Form API field and a "file" field for Field module.
FileCopyTest.php in core/tests/Drupal/KernelTests/Core/File/FileCopyTest.php
FileRepositoryTest.php in core/modules/file/tests/src/Kernel/FileRepositoryTest.php
FileSystem.php in core/lib/Drupal/Core/File/FileSystem.php
FileSystemTest.php in core/tests/Drupal/KernelTests/Core/File/FileSystemTest.php
FileUpload.php in core/modules/jsonapi/src/Controller/FileUpload.php
FileUploadHandler.php in core/modules/file/src/Upload/FileUploadHandler.php
FileUploadResource.php in core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
MoveTest.php in core/modules/file/tests/src/Kernel/MoveTest.php

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