resource icon is not shown when saving it with custom extension

### Tested versions

v4.3.beta1.mono.official [a4f2ea91a]

### System information

Windows 11

### Issue description

I build my custom resource loader and saver to have a custom extension.
When I now save my resource by the custom extension, the resource icon is not shown in the filesystem.


### Steps to reproduce

Use the attached project and create a new resource from the `GeometryNodeGraphResource`
- do save as `graph.npgh`
- do save as `graph.tres`
You can see the icon is only shown for the *.tres extension.

## Additional Info
On v4.2.2.stable.official [15073afe3] the custom resource load save is completely broken :(


### Minimal reproduction project (MRP)

[fileicon.zip](https://github.com/user-attachments/files/15910660/fileicon.zip)

Start with the attached fileicon.zip minimal reproduction project and reproduce saving GeometryNodeGraphResource as graph.npgh versus graph.tres on Windows. Trace the editor filesystem icon handling for custom resource extensions; done means the custom-extension resource shows the same icon as the corresponding .tres resource without regressing custom resource loading and saving.

Written by the indexing model from the issue text.