24 files declare their use of FileInterface

BaseFileConstraintValidator.php in core/modules/file/src/Plugin/Validation/Constraint/BaseFileConstraintValidator.php
claro.theme in core/themes/claro/claro.theme
Functions to support theming in the Claro theme.
DatabaseFileUsageBackend.php in core/modules/file/src/FileUsage/DatabaseFileUsageBackend.php
EditorFileReference.php in core/modules/editor/src/Plugin/Filter/EditorFileReference.php
EntityReferenceFieldItemNormalizer.php in core/modules/serialization/src/Normalizer/EntityReferenceFieldItemNormalizer.php
file.api.php in core/modules/file/file.api.php
file.module in core/modules/file/file.module
File.php in core/modules/file/src/Entity/File.php
File.php in core/modules/media/src/Plugin/media/Source/File.php
FileHooks.php in core/modules/file/src/Hook/FileHooks.php
FileSelection.php in core/modules/file/src/Plugin/EntityReferenceSelection/FileSelection.php
FileUploadForm.php in core/modules/media_library/src/Form/FileUploadForm.php
FileUploadResult.php in core/modules/file/src/Upload/FileUploadResult.php
FileUsageBase.php in core/modules/file/src/FileUsage/FileUsageBase.php
FileUsageInterface.php in core/modules/file/src/FileUsage/FileUsageInterface.php
FileValidationEvent.php in core/modules/file/src/Validation/FileValidationEvent.php
FileValidator.php in core/modules/file/src/Validation/FileValidator.php
FileValidatorInterface.php in core/modules/file/src/Validation/FileValidatorInterface.php
ImageHooks.php in core/modules/image/src/Hook/ImageHooks.php
Importer.php in core/lib/Drupal/Core/DefaultContent/Importer.php
locale.bulk.inc in core/modules/locale/locale.bulk.inc
ResponsiveImageFormatter.php in core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php
Status.php in core/modules/file/src/Plugin/views/filter/Status.php
UrlPlainFormatter.php in core/modules/file/src/Plugin/Field/FieldFormatter/UrlPlainFormatter.php