feat: add MIME plumbing for markdown (.md) format

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/mime.test.js

@@ -20,6 +20,7 @@ describe('outputMimeType', () => {
     expect(outputMimeType('docx')).toBe(
       'application/vnd.openxmlformats-officedocument.wordprocessingml.document'
     );
+    expect(outputMimeType('md')).toBe('text/markdown');
   });
 
   it('throws for an unknown target format', () => {
@@ -61,6 +62,17 @@ describe('resolveInputFormat', () => {
     await fs.unlink(fixturePath);
   });
 
+  it('trusts the declared format for undetectable md files', async () => {
+    const fixturePath = path.join(import.meta.dirname, 'fixtures', 'sample.md');
+    await fs.writeFile(fixturePath, '# plain markdown, no magic bytes');
+
+    const result = await resolveInputFormat(fixturePath, 'md');
+
+    expect(result).toEqual({ mime: 'text/markdown', valid: true });
+
+    await fs.unlink(fixturePath);
+  });
+
   it('rejects an undetectable file declared as a binary format', async () => {
     const fixturePath = path.join(import.meta.dirname, 'fixtures', 'sample-fake.png');
     await fs.writeFile(fixturePath, 'this is not really a PNG');