Ignore automatic nesting for Designer files

I'm currently using the filenesting for C# projects. Everything works fine except that .Designer.cs files are automatically nested.

Background: We are having CUDA-files with the cu extensions. A process creates three additional files. Aas an example I have named my CUDA file Convolution.cu. So what is created is

```
Convolution.cu
+ Convolution.cs
+ Convolution.Designer.cs
+ Convolution.ptx
```

which is also the hierarchy I expect in the solution explorer.
I have defined my own file nesting so that I don't have to use the complex DependentUpon project entries:

```
{
"help": "https://go.microsoft.com/fwlink/?linkid=866610",
"root": true,

"dependentFileProviders": {
"add": {
"extensionToExtension": {
"add": {
".cs": [
".cu"
],
".ptx": [
".cu"
]
}
}
}
}
}
```

What I get is this:

```
Convolution.cu
+ Convolution.cs
+ Convolution.Designer.cs
+ Convolution.ptx

```

Obviously some other mechanism is already active that does the nesting.

I search a while but there is obviously not Default filenesting.json I can have a look at.
I expect that this mechanism is not done by filenesting.json but some other method?

Is there any way to remove the nesting?