Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: 1.86.2
• OS Version: Not relevant

Steps to Reproduce:

1. Create a new file
2. Press CTRL SHIFT P
3. Select Change Language Mode and choose YAML
4. Press CTRL S
5. Notice that the extension is .yml

YAML's documentation recommends the .yaml extension: see https://yaml.org/faq