diff --git a/backend/go.mod b/backend/go.mod
index a3f7dc7..10ac13a 100644
--- a/backend/go.mod
+++ b/backend/go.mod
@@ -38,6 +38,7 @@ require (
 	github.com/modern-go/reflect2 v1.0.2 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
 	github.com/rogpeppe/go-internal v1.14.1 // indirect
+	github.com/rwcarlsen/goexif v0.0.0-20190401172101-9e8deecbddbd // indirect
 	github.com/sethvargo/go-retry v0.3.0 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.3.1 // indirect
diff --git a/backend/go.sum b/backend/go.sum
index 246d912..02c8a8f 100644
--- a/backend/go.sum
+++ b/backend/go.sum
@@ -80,6 +80,8 @@ github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qqbHyh8v60DhA7CoWK5oRCqLrMHRGoxYCSS9EjAz6Eo=
 github.com/rogpeppe/go-internal v1.14.1 h1:UQB4HGPB6osV0SQTLymcB4TgvyWu6ZyliaW0tI/otEQ=
 github.com/rogpeppe/go-internal v1.14.1/go.mod h1:MaRKkUm5W0goXpeCfT7UZI6fk/L7L7so1lCWt35ZSgc=
+github.com/rwcarlsen/goexif v0.0.0-20190401172101-9e8deecbddbd h1:CmH9+J6ZSsIjUK3dcGsnCnO41eRBOnY12zwkn5qVwgc=
+github.com/rwcarlsen/goexif v0.0.0-20190401172101-9e8deecbddbd/go.mod h1:hPqNNc0+uJM6H+SuU8sEs5K5IQeKccPqeSjfgcKGgPk=
 github.com/sethvargo/go-retry v0.3.0 h1:EEt31A35QhrcRZtrYFDTBg91cqZVnFL2navjDrah2SE=
 github.com/sethvargo/go-retry v0.3.0/go.mod h1:mNX17F0C/HguQMyMyJxcnU471gOZGxCLyYaFyAZraas=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
diff --git a/backend/internal/service/file_service.go b/backend/internal/service/file_service.go
new file mode 100644
index 0000000..5332e78
--- /dev/null
+++ b/backend/internal/service/file_service.go
@@ -0,0 +1,392 @@
+package service
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/rwcarlsen/goexif/exif"
+
+	"tanabata/backend/internal/domain"
+	"tanabata/backend/internal/port"
+)
+
+const fileObjectType = "file"
+
+// fileObjectTypeID is the primary key of the "file" row in core.object_types.
+// It matches the first value inserted in 007_seed_data.sql.
+const fileObjectTypeID int16 = 1
+
+// UploadParams holds the parameters for uploading a new file.
+type UploadParams struct {
+	Reader       io.Reader
+	MIMEType     string
+	OriginalName *string
+	Notes        *string
+	Metadata     json.RawMessage
+	IsPublic     bool
+	TagIDs       []uuid.UUID
+}
+
+// UpdateParams holds the parameters for updating file metadata.
+type UpdateParams struct {
+	OriginalName    *string
+	Notes           *string
+	Metadata        json.RawMessage
+	ContentDatetime *time.Time
+	IsPublic        *bool
+	TagIDs          *[]uuid.UUID // nil means don't change tags
+}
+
+// FileService handles business logic for file records.
+type FileService struct {
+	files   port.FileRepo
+	mimes   port.MimeRepo
+	storage port.FileStorage
+	acl     *ACLService
+	audit   *AuditService
+	tx      port.Transactor
+}
+
+// NewFileService creates a FileService.
+func NewFileService(
+	files port.FileRepo,
+	mimes port.MimeRepo,
+	storage port.FileStorage,
+	acl *ACLService,
+	audit *AuditService,
+	tx port.Transactor,
+) *FileService {
+	return &FileService{
+		files:   files,
+		mimes:   mimes,
+		storage: storage,
+		acl:     acl,
+		audit:   audit,
+		tx:      tx,
+	}
+}
+
+// Upload validates the MIME type, saves the file to storage, creates the DB
+// record, and applies any initial tags — all within a single transaction.
+func (s *FileService) Upload(ctx context.Context, p UploadParams) (*domain.File, error) {
+	userID, _, _ := domain.UserFromContext(ctx)
+
+	// Validate MIME type against the whitelist.
+	mime, err := s.mimes.GetByName(ctx, p.MIMEType)
+	if err != nil {
+		return nil, err // ErrUnsupportedMIME or DB error
+	}
+
+	// Buffer the upload so we can extract EXIF without re-reading storage.
+	var buf bytes.Buffer
+	if _, err := io.Copy(&buf, p.Reader); err != nil {
+		return nil, fmt.Errorf("FileService.Upload: read body: %w", err)
+	}
+	data := buf.Bytes()
+
+	// Extract EXIF metadata (best-effort; non-image files will error silently).
+	exifData := extractEXIF(data)
+
+	// Assign UUID v7 so CreatedAt can be derived from it later.
+	fileID, err := uuid.NewV7()
+	if err != nil {
+		return nil, fmt.Errorf("FileService.Upload: generate UUID: %w", err)
+	}
+
+	// Save file bytes to disk before opening the transaction so that a disk
+	// failure does not abort an otherwise healthy DB transaction.
+	if _, err := s.storage.Save(ctx, fileID, bytes.NewReader(data)); err != nil {
+		return nil, fmt.Errorf("FileService.Upload: save to storage: %w", err)
+	}
+
+	var created *domain.File
+	txErr := s.tx.WithTx(ctx, func(ctx context.Context) error {
+		f := &domain.File{
+			ID:           fileID,
+			OriginalName: p.OriginalName,
+			MIMEType:     mime.Name,
+			MIMEExtension: mime.Extension,
+			Notes:        p.Notes,
+			Metadata:     p.Metadata,
+			EXIF:         exifData,
+			CreatorID:    userID,
+			IsPublic:     p.IsPublic,
+		}
+
+		var createErr error
+		created, createErr = s.files.Create(ctx, f)
+		if createErr != nil {
+			return createErr
+		}
+
+		if len(p.TagIDs) > 0 {
+			if err := s.files.SetTags(ctx, created.ID, p.TagIDs); err != nil {
+				return err
+			}
+			// Re-fetch to populate Tags on the returned value.
+			tags, err := s.files.ListTags(ctx, created.ID)
+			if err != nil {
+				return err
+			}
+			created.Tags = tags
+		}
+		return nil
+	})
+	if txErr != nil {
+		// Attempt to clean up the orphaned file; ignore cleanup errors.
+		_ = s.storage.Delete(ctx, fileID)
+		return nil, txErr
+	}
+
+	objType := fileObjectType
+	_ = s.audit.Log(ctx, "file_create", &objType, &created.ID, nil)
+	return created, nil
+}
+
+// Get returns a file by ID, enforcing view ACL.
+func (s *FileService) Get(ctx context.Context, id uuid.UUID) (*domain.File, error) {
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+
+	f, err := s.files.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+
+	ok, err := s.acl.CanView(ctx, userID, isAdmin, f.CreatorID, f.IsPublic, fileObjectTypeID, id)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, domain.ErrForbidden
+	}
+	return f, nil
+}
+
+// Update applies metadata changes to a file, enforcing edit ACL.
+func (s *FileService) Update(ctx context.Context, id uuid.UUID, p UpdateParams) (*domain.File, error) {
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+
+	f, err := s.files.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+
+	ok, err := s.acl.CanEdit(ctx, userID, isAdmin, f.CreatorID, fileObjectTypeID, id)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, domain.ErrForbidden
+	}
+
+	patch := &domain.File{}
+	if p.OriginalName != nil {
+		patch.OriginalName = p.OriginalName
+	}
+	if p.Notes != nil {
+		patch.Notes = p.Notes
+	}
+	if p.Metadata != nil {
+		patch.Metadata = p.Metadata
+	}
+	if p.ContentDatetime != nil {
+		patch.ContentDatetime = *p.ContentDatetime
+	}
+	if p.IsPublic != nil {
+		patch.IsPublic = *p.IsPublic
+	}
+
+	var updated *domain.File
+	txErr := s.tx.WithTx(ctx, func(ctx context.Context) error {
+		var updateErr error
+		updated, updateErr = s.files.Update(ctx, id, patch)
+		if updateErr != nil {
+			return updateErr
+		}
+		if p.TagIDs != nil {
+			if err := s.files.SetTags(ctx, id, *p.TagIDs); err != nil {
+				return err
+			}
+			tags, err := s.files.ListTags(ctx, id)
+			if err != nil {
+				return err
+			}
+			updated.Tags = tags
+		}
+		return nil
+	})
+	if txErr != nil {
+		return nil, txErr
+	}
+
+	objType := fileObjectType
+	_ = s.audit.Log(ctx, "file_edit", &objType, &id, nil)
+	return updated, nil
+}
+
+// Delete soft-deletes a file (moves to trash), enforcing edit ACL.
+func (s *FileService) Delete(ctx context.Context, id uuid.UUID) error {
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+
+	f, err := s.files.GetByID(ctx, id)
+	if err != nil {
+		return err
+	}
+
+	ok, err := s.acl.CanEdit(ctx, userID, isAdmin, f.CreatorID, fileObjectTypeID, id)
+	if err != nil {
+		return err
+	}
+	if !ok {
+		return domain.ErrForbidden
+	}
+
+	if err := s.files.SoftDelete(ctx, id); err != nil {
+		return err
+	}
+
+	objType := fileObjectType
+	_ = s.audit.Log(ctx, "file_delete", &objType, &id, nil)
+	return nil
+}
+
+// Restore moves a soft-deleted file out of trash, enforcing edit ACL.
+func (s *FileService) Restore(ctx context.Context, id uuid.UUID) (*domain.File, error) {
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+
+	f, err := s.files.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+
+	ok, err := s.acl.CanEdit(ctx, userID, isAdmin, f.CreatorID, fileObjectTypeID, id)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, domain.ErrForbidden
+	}
+
+	restored, err := s.files.Restore(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+
+	objType := fileObjectType
+	_ = s.audit.Log(ctx, "file_restore", &objType, &id, nil)
+	return restored, nil
+}
+
+// PermanentDelete removes the file record and its stored bytes. Only allowed
+// when the file is already in trash. Restricted to admins and the creator.
+func (s *FileService) PermanentDelete(ctx context.Context, id uuid.UUID) error {
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+
+	f, err := s.files.GetByID(ctx, id)
+	if err != nil {
+		return err
+	}
+	if !f.IsDeleted {
+		return domain.ErrValidation
+	}
+
+	ok, err := s.acl.CanEdit(ctx, userID, isAdmin, f.CreatorID, fileObjectTypeID, id)
+	if err != nil {
+		return err
+	}
+	if !ok {
+		return domain.ErrForbidden
+	}
+
+	if err := s.files.DeletePermanent(ctx, id); err != nil {
+		return err
+	}
+	_ = s.storage.Delete(ctx, id)
+
+	objType := fileObjectType
+	_ = s.audit.Log(ctx, "file_permanent_delete", &objType, &id, nil)
+	return nil
+}
+
+// Replace swaps the stored bytes for a file with new content. The MIME type
+// may change. Thumbnail/preview caches are not invalidated here — callers
+// should handle that if needed.
+func (s *FileService) Replace(ctx context.Context, id uuid.UUID, p UploadParams) (*domain.File, error) {
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+
+	f, err := s.files.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+
+	ok, err := s.acl.CanEdit(ctx, userID, isAdmin, f.CreatorID, fileObjectTypeID, id)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, domain.ErrForbidden
+	}
+
+	mime, err := s.mimes.GetByName(ctx, p.MIMEType)
+	if err != nil {
+		return nil, err
+	}
+
+	var buf bytes.Buffer
+	if _, err := io.Copy(&buf, p.Reader); err != nil {
+		return nil, fmt.Errorf("FileService.Replace: read body: %w", err)
+	}
+	data := buf.Bytes()
+	exifData := extractEXIF(data)
+
+	// Save new bytes, overwriting the existing stored file.
+	if _, err := s.storage.Save(ctx, id, bytes.NewReader(data)); err != nil {
+		return nil, fmt.Errorf("FileService.Replace: save to storage: %w", err)
+	}
+
+	patch := &domain.File{
+		MIMEType:      mime.Name,
+		MIMEExtension: mime.Extension,
+		EXIF:          exifData,
+	}
+	if p.OriginalName != nil {
+		patch.OriginalName = p.OriginalName
+	}
+
+	updated, err := s.files.Update(ctx, id, patch)
+	if err != nil {
+		return nil, err
+	}
+
+	objType := fileObjectType
+	_ = s.audit.Log(ctx, "file_replace", &objType, &id, nil)
+	return updated, nil
+}
+
+// List delegates to FileRepo with the given params.
+func (s *FileService) List(ctx context.Context, params domain.FileListParams) (*domain.FilePage, error) {
+	return s.files.List(ctx, params)
+}
+
+// ---------------------------------------------------------------------------
+// Internal helpers
+// ---------------------------------------------------------------------------
+
+// extractEXIF attempts to parse EXIF data from raw bytes and marshal it to
+// JSON. Returns nil on any error (non-image files, no EXIF header, etc.).
+func extractEXIF(data []byte) json.RawMessage {
+	x, err := exif.Decode(bytes.NewReader(data))
+	if err != nil {
+		return nil
+	}
+	b, err := x.MarshalJSON()
+	if err != nil {
+		return nil
+	}
+	return json.RawMessage(b)
+}
\ No newline at end of file