diff --git a/frontend/test/common/universalVaultFormat.spec.ts b/frontend/test/common/universalVaultFormat.spec.ts
index 06d4f89d..90cdb704 100644
--- a/frontend/test/common/universalVaultFormat.spec.ts
+++ b/frontend/test/common/universalVaultFormat.spec.ts
@@ -1,5 +1,6 @@
 import { use as chaiUse, expect } from 'chai';
 import chaiAsPromised from 'chai-as-promised';
+import chaiBytes from 'chai-bytes';
 import { before, describe } from 'mocha';
 import { base64 } from 'rfc4648';
 import { VaultDto } from '../../src/common/backend';
@@ -8,6 +9,7 @@ import { JsonJWE } from '../../src/common/jwe';
 import { MemberKey, RecoveryKey, UniversalVaultFormat, VaultMetadata } from '../../src/common/universalVaultFormat';
 
 chaiUse(chaiAsPromised);
+chaiUse(chaiBytes);
 
 // key coordinates from MDN examples:
 const alicePublic: JsonWebKey = {
@@ -287,6 +289,7 @@ describe('UVF', () => {
         const rootDirId = base64.parse('24UBEDeGu5taq7U4GqyA0MXUXb9HTYS6p3t9vvHGJAc=');
         const fileContent = await uvf.encryptFile(rootDirId, uvf.metadata.initialSeedId);
         expect(fileContent).to.have.a.lengthOf(128);
+        expect(fileContent.slice(0, 4)).to.equalBytes(new Uint8Array([0x75, 0x76, 0x66, 0x01])); // magic bytes
       });
     });
   });