4.1 Note Files ↑
4.1.4 Note File Extensions
Index: file extensions;
Any of the following file extensions may be used:
The two preferred extensions are ‘.txt’ (short for text) and ‘.md’ (short for Markdown).
When you invoke the Text Edit Note command under the Note menu to open the currently displayed Note in your default text editor, your choice of file extension will determine the selection of the app to be opened, based on your current Info settings for that file type.