How to pass .md extension files to Copilot's file context

[itou-rui]

First of all, thank you for your work on this great plugin.

I'm using CopilotChat.nvim on Neovim (macOS), and I'm having trouble adding Markdown files as context.
Files with the .md extension are not recognized as context, while files with the .markdown extension work as expected.

Even when I use #file:README.md, Copilot does not receive README.md.
However, using #file:README.markdown works.

When I run set filetype? with a .md file open, it returns markdown.

If anyone has encountered this issue or knows a solution, I would greatly appreciate your advice.