[Feature] add more extentions

[SeanYHwang]

I am trying to add a .mdx file as a Markdown file, but the extension is fixed with .md.
Thus, it would be good if we can select multiple extensions or custom like:

.txt, .md, .mdx, .markdown

[abnerlee]

What's your operating system?

[ghenry]

On macOS I set custom extension to pml, but my pml files don't show. Works fine on Linux. Any ideas?