diff --git a/packages/hydrooj/src/model/storage.ts b/packages/hydrooj/src/model/storage.ts
index 125d0473..f6ab1987 100644
--- a/packages/hydrooj/src/model/storage.ts
+++ b/packages/hydrooj/src/model/storage.ts
@@ -14,7 +14,7 @@ export class StorageModel {
     static coll = db.collection('storage');
 
     static generateId(ext: string) {
-        return `${nanoid(3).replace(/[_-]/g, '0')}/${nanoid().replace(/[_-]/g, '0')}${ext}`;
+        return `${nanoid(3).replace(/[_-]/g, '0')}/${nanoid().replace(/[_-]/g, '0')}${ext}`.toLowerCase();
     }
 
     static async put(path: string, file: string | Buffer | Readable, owner?: number) {
@@ -106,10 +106,10 @@ export class StorageModel {
         meta['Content-Type'] = (dst.endsWith('.ans') || dst.endsWith('.out'))
             ? 'text/plain'
             : lookup(dst) || 'application/octet-stream';
-        let _id = `${nanoid(3)}/${nanoid()}${extname(dst)}`;
+        let _id = StorageModel.generateId(extname(dst));
         // Make sure id is not used
         // eslint-disable-next-line no-await-in-loop
-        while (await StorageModel.coll.findOne({ _id })) _id = `${nanoid(3)}/${nanoid()}${extname(dst)}`;
+        while (await StorageModel.coll.findOne({ _id })) _id = StorageModel.generateId(extname(dst));
         const result = await storage.copy(value._id, dst);
         const { metaData, size, etag } = await storage.getMeta(_id);
         await StorageModel.coll.insertOne({