fix(admin,server): 上传改用 file.buffer 并移除手写 multipart 头

apps/admin/src/pages/ClassroomRentals/index.tsx

@@ -140,9 +140,7 @@ const ClassroomRentalsPage: React.FC = () => {
   );
   const uploadContractMutation = useApiMutation(
     async ({ id, formData }: { id: number; formData: FormData }) =>
-      api.post(`/classroom-rentals/${id}/contract`, formData, {
-        headers: { 'Content-Type': 'multipart/form-data' },
-      }),
+      api.post(`/classroom-rentals/${id}/contract`, formData),
     { invalidate: [['classroom-rentals']] },
   );