To add “Markdown File” item from Context Menu → New in explorer. You can download and open this reg file.
Or, create an txt file, adding following content, rename it to *.reg, then click it to register its content.
Windows Registry Editor Version 5.00 [HKEY_CLASSES_ROOT\.md] @="markdown" [HKEY_CLASSES_ROOT\.md\ShellNew] "NullFile"="" [HKEY_CLASSES_ROOT\markdown] @="Blank Markdown file"
You can use 3rd tools for this to gave a “New File” item in context menu in Finder.