@icon annotation for custom resources not working with custom extensions

### Tested versions

- 4.3 rc1 (Windows 11)

### System information

Godot v4.3.rc1 - Windows 10.0.22621 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3080 Ti (NVIDIA; 32.0.15.6070) - AMD Ryzen 9 5950X 16-Core Processor (32 Threads)

### Issue description

When using @ icon annotation to set an icon for a custom resource, it does not show in the editor's filesystem if the resource has a custom extension (not .tres)

### Steps to reproduce

1. Open MRP.
2. Open FileSystem.
3. Open the res_1.new file.
4. Notice the icon loads correctly in the inspector, but not the FileSystem.

I attached a screenshot too.

### Minimal reproduction project (MRP)

[icon-extension-mrp.zip](https://github.com/user-attachments/files/16433782/icon-extension-mrp.zip)

Open the attached icon-extension-mrp.zip and reproduce the mismatch by checking res_1.new in the editor's FileSystem and Inspector. Start by tracing how the FileSystem displays custom resource icons versus how the Inspector resolves the @icon annotation. Done means the annotated icon appears for custom-resource files with non-.tres extensions, while existing Inspector behavior remains correct.

Written by the indexing model from the issue text.