.txt File Extension

I find that MacDown opens .txt files and recognizes markdown formatting just fine, but when you save a new document .md is always appended. I think it would be generally helpful to have the option to choose your own extension.
The way I use a lot of text across applications and devices makes keeping the more common .txt extension consistent, and I think a lot of other users would too.

Start by tracing MacDown's new-document save flow and how it assigns the .md extension, then inspect the existing save UI for a suitable place to choose an extension. Done means a new document can be saved with .txt or another user-selected extension without automatically appending .md.

Written by the indexing model from the issue text.