diff --git a/backend/src/routes/files.ts b/backend/src/routes/files.ts
index c0a1107..01e6e17 100644
--- a/backend/src/routes/files.ts
+++ b/backend/src/routes/files.ts
@@ -65,7 +65,8 @@ router.post('/upload/:taskId', upload.single('file'), async (req, res, next) =>
 // GET /api/files/:id/view  — 파일 미리보기 (브라우저에서 바로 열기)
 router.get('/:id/view', async (req, res, next) => {
   try {
-    const file = await prisma.file.findUnique({ where: { id: req.params.id } });
+    const fileId = String(req.params.id);
+    const file = await prisma.file.findUnique({ where: { id: fileId } });
     if (!file) throw new AppError(404, '파일을 찾을 수 없습니다.');
     if (!fs.existsSync(file.path)) throw new AppError(404, '파일이 서버에 없습니다.');
 
@@ -80,7 +81,8 @@ router.get('/:id/view', async (req, res, next) => {
 // GET /api/files/:id/download  — 파일 다운로드
 router.get('/:id/download', async (req, res, next) => {
   try {
-    const file = await prisma.file.findUnique({ where: { id: req.params.id } });
+    const fileId = String(req.params.id);
+    const file = await prisma.file.findUnique({ where: { id: fileId } });
     if (!file) throw new AppError(404, '파일을 찾을 수 없습니다.');
     if (!fs.existsSync(file.path)) throw new AppError(404, '파일이 서버에 없습니다.');
 
@@ -96,7 +98,8 @@ router.post('/:id/replace', upload.single('file'), async (req, res, next) => {
   try {
     if (!req.file) throw new AppError(400, '파일이 없습니다.');
 
-    const existing = await prisma.file.findUnique({ where: { id: req.params.id } });
+    const fileId = String(req.params.id);
+    const existing = await prisma.file.findUnique({ where: { id: fileId } });
     if (!existing) throw new AppError(404, '파일을 찾을 수 없습니다.');
 
     if (fs.existsSync(existing.path)) {
@@ -104,7 +107,7 @@ router.post('/:id/replace', upload.single('file'), async (req, res, next) => {
     }
 
     const file = await prisma.file.update({
-      where: { id: req.params.id },
+      where: { id: fileId },
       data: {
         filename: req.file.filename,
         originalName: fixOriginalName(req.file.originalname),
@@ -123,12 +126,13 @@ router.post('/:id/replace', upload.single('file'), async (req, res, next) => {
 // PATCH /api/files/:id  — 표시명·순서 수정
 router.patch('/:id', async (req, res, next) => {
   try {
+    const fileId = String(req.params.id);
     const { displayName, sortOrder } = req.body as { displayName?: string; sortOrder?: number };
-    const existing = await prisma.file.findUnique({ where: { id: req.params.id } });
+    const existing = await prisma.file.findUnique({ where: { id: fileId } });
     if (!existing) throw new AppError(404, '파일을 찾을 수 없습니다.');
 
     const file = await prisma.file.update({
-      where: { id: req.params.id },
+      where: { id: fileId },
       data: {
         ...(displayName !== undefined && { displayName: displayName?.trim() || null }),
         ...(sortOrder !== undefined && { sortOrder: Number(sortOrder) }),
@@ -144,7 +148,8 @@ router.patch('/:id', async (req, res, next) => {
 // DELETE /api/files/:id  — 파일 삭제
 router.delete('/:id', async (req, res, next) => {
   try {
-    const file = await prisma.file.findUnique({ where: { id: req.params.id } });
+    const fileId = String(req.params.id);
+    const file = await prisma.file.findUnique({ where: { id: fileId } });
     if (!file) throw new AppError(404, '파일을 찾을 수 없습니다.');
 
     // 실제 파일 삭제
@@ -152,7 +157,7 @@ router.delete('/:id', async (req, res, next) => {
       fs.unlinkSync(file.path);
     }
 
-    await prisma.file.delete({ where: { id: req.params.id } });
+    await prisma.file.delete({ where: { id: fileId } });
     res.status(204).send();
   } catch (err) {
     next(err);