Namespace files structure doesn't starts with a leading '/'. Inconsistency with storage's current structure
### Explain the bug

See [here](https://user-images.githubusercontent.com/37618489/281166071-ab889fc4-0c05-46c9-830f-5ed1bf2640a0.png)

### Environment Information

- Kestra Version:
- Operating System and Java Version (if not using Kestra Docker image):

core/src/main/java/io/kestra/core/storages/StorageInterface.java
core/src/main/java/io/kestra/core/storages/StorageInterface.java
core/src/test/java/io/kestra/core/storage/StorageInterfaceTest.java
diff --git a/core/src/main/java/io/kestra/core/storages/StorageInterface.java b/core/src/main/java/io/kestra/core/storages/StorageInterface.java
index 2d2ab16b1d..9ebeb93b57 100644
--- a/core/src/main/java/io/kestra/core/storages/StorageInterface.java
+++ b/core/src/main/java/io/kestra/core/storages/StorageInterface.java
@@ -163,10 +163,10 @@ default String cachePrefix(String namespace, String flowId, String taskId, @Null
     }
 
     default String namespaceFilePrefix(String namespace) {
-        return String.join("/", List.of(
+        return fromParts(
             namespace,
             "files"
-        ));
+        );
     }
 
     default Optional<String> extractExecutionId(URI path) {

diff --git a/core/src/test/java/io/kestra/core/storage/StorageInterfaceTest.java b/core/src/test/java/io/kestra/core/storage/StorageInterfaceTest.java
index 914d1068eb..271ff95162 100644
--- a/core/src/test/java/io/kestra/core/storage/StorageInterfaceTest.java
+++ b/core/src/test/java/io/kestra/core/storage/StorageInterfaceTest.java
@@ -86,6 +86,6 @@ void outputPrefix() {
     void namespaceFilePrefix() {
         var prefix = storageInterface.namespaceFilePrefix("io.namespace");
         assertThat(prefix, notNullValue());
-        assertThat(prefix, is("io.namespace/files"));
+        assertThat(prefix, is("/io.namespace/files"));
     }
 }
