12 files declare their use of FileInterface in Zircon Profile 8

DatabaseFileUsageBackend.php in core/modules/file/src/FileUsage/DatabaseFileUsageBackend.php

Contains \Drupal\file\FileUsage\DatabaseFileUsageBackend.

file.module in core/modules/file/file.module

Defines a "managed_file" Form API field and a "file" field for Field module.

File.php in core/modules/file/src/Entity/File.php

Contains \Drupal\file\Entity\File.

FileFieldTestBase.php in core/modules/file/src/Tests/FileFieldTestBase.php

Contains \Drupal\file\Tests\FileFieldTestBase.

FileManagedTestBase.php in core/modules/file/src/Tests/FileManagedTestBase.php

Contains \Drupal\file\Tests\FileManagedTestBase.

FileManagedUnitTestBase.php in core/modules/file/src/Tests/FileManagedUnitTestBase.php

Contains \Drupal\file\Tests\FileManagedUnitTestBase.

FileUsageBase.php in core/modules/file/src/FileUsage/FileUsageBase.php

Contains \Drupal\file\FileUsage\FileUsageBase.

FileUsageInterface.php in core/modules/file/src/FileUsage/FileUsageInterface.php

Contains \Drupal\file\FileUsage\FileUsageInterface.

locale.bulk.inc in core/modules/locale/locale.bulk.inc

Mass import-export and batch import functionality for Gettext .po files.

MigrateFileTest.php in core/modules/file/src/Tests/Migrate/d6/MigrateFileTest.php

Contains \Drupal\file\Tests\Migrate\d6\MigrateFileTest.

MigrateFileTest.php in core/modules/file/src/Tests/Migrate/d7/MigrateFileTest.php

Contains \Drupal\file\Tests\Migrate\d7\MigrateFileTest.

TestFileUsage.php in core/modules/system/tests/modules/service_provider_test/src/TestFileUsage.php

Contains \Drupal\service_provider_test\TestFileUsage.