Markdown file associations break Copilot instructions

[atachh]

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: 1.101.0 (Universal), Commit: dfaf441
• OS Version: MacOS 15.3.2 (24D81)

Steps to Reproduce:

1. Open User Settings with cmd+P -> Preferences: Open User Settings (JSON)
2. Add the following to your settings: "files.associations": { "*.md": "markdown" }
3. Create a Copilot Instructions file with cmd+P -> Chat: New Instructions File
4. Make sure that applyTo: '**' is present in the Frontmatter, write anything in the instructions file itself
5. Create any file and save it
6. Open the Copilot Chat and add the file created in step 4 to the context, then write your prompt and press send

Expected result:
The instructions file is attached to the request

Actual result:
The instructions file is ignored