Enable opening non md files

[hekatonsure]

Problem

Current implementation force-appends .md to all paths, preventing users from opening PDFs, images, or other supported file types.

Solution

Check for existing file extension before defaulting to .md. Simple regex test preserves backward compatibility while enabling broader file support.

Changes

• Add extension detection via /\.[^.]+$/ regex
• Only append .md when no extension present
• Fixed template literal syntax in error notices

Tested with: note.md, note, image.png, doc.pdf

Limitations

Doesn't auto-resolve to existing non-md files when extension omitted. E.g., openNote('myfile') won't find myfile.pdf - still defaults to searching for myfile.md. Full path with extension required for non-markdown files, which may be preferred use pattern.