How to reproduce:
1. Open the “SceneTemplateBug“
2. Open the New Scene window (File > New Scene)
3. Select “MyTemplate” then select “Create”
4. Assign any name to the new Scene and save the file in the Project folder

How to reproduce in new project:
1. Create a new project with the “3D Core” template
2. Add a Cube GameObject (GameObject > 3D Object > Cube)
3. Turn the “Cube” GameObject into a Prefab and save the Scene (drag and drop “Cube” GameObject to Project)
4. Create a new Scene Template from “SampleScene” (Project > Assets > right-click “SampleScene” > Create > Scene Template From Scene)
5. Open the New Scene window (File > New Scene)
6. Select “SampleScene” then select “Create”
7. Assign any name to the new Scene and save the file in the Project folder

Expected result: new Scene is created
Actual result: new Scene is not created and the “Incorrect extension. Required extension is: '.scenetemplate'.” warning is logged to the Console

Reproduced with: 2021.3.28f1
Not reproduced with: 2021.3.27f1, 2022.3.5f1, 2023.1.4f1, 2023.2.0a22

Reproduced on: Windows 10 (by reporter), Windows 11

Notes:
1. On Windows 11, when prompted to save the new Scene, selecting “Save as type” as “All files (*.*)” and renaming the extension of the file to “.scenetemplate” saves an unworking Prefab-like scene and throws an error → full error is provided in internal comments
2. Not reproducible when creating a new Scene from a Template that does not have a Prefab