Netbeans 21 IDE hides file extension in the Projects explorer

### Apache NetBeans version

Apache NetBeans 21

### What happened

When opening the projects of files view pane, the extension of java files is hidden.
It is done inconsistently, as some files do have the extension and others do not.
see the screenshot below of the projects explorer.


### Language / Project Type / NetBeans Component

Netbeans Projects explorer

### How to reproduce

Unsure, after a restart of the IDE, the problem is gone.

### Did this work correctly in an earlier version?

No / Don't know

### Operating System

Linux Ubuntu 22.04 LTS

### JDK

openjdk version "21.0.1" 2023-10-17 LTS

### Apache NetBeans packaging

Apache NetBeans binary zip

### Anything else

No

### Are you willing to submit a pull request?

No

Start by trying to reproduce the inconsistent Java file-extension display in the Projects explorer with NetBeans 21 on Ubuntu 22.04 and JDK 21. The issue provides no source file or test entry point; done means identifying the cause and ensuring Java file extensions display consistently after opening or restarting the IDE.

Written by the indexing model from the issue text.