It is not an unnecessary extension, and .txt and .md should not be treated the same. Markdown files should have the opinionated ruleset of markdown, including its syntax and validation within the editor, which can be enforced by detection of the .md extension. Text files on the other hand require none of that, and should be rendered purely as plain text without syntax highlighting or validation as Markdown has. While MD is plain text from a technical standpoint, it is not treated as such from an editor or UX standpoint.
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