feat: add MIME sniffing and output MIME lookup

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

test/mime.test.js

@@ -0,0 +1,27 @@
+import { describe, it, expect } from 'vitest';
+import path from 'node:path';
+import { detectInputMime, outputMimeType } from '../src/mime.js';
+
+describe('detectInputMime', () => {
+  it('detects PNG from magic bytes regardless of file extension', async () => {
+    const fixturePath = path.join(import.meta.dirname, 'fixtures', 'sample.png');
+
+    const result = await detectInputMime(fixturePath);
+
+    expect(result).toEqual({ ext: 'png', mime: 'image/png' });
+  });
+});
+
+describe('outputMimeType', () => {
+  it('returns the MIME type for a known target format', () => {
+    expect(outputMimeType('pdf')).toBe('application/pdf');
+    expect(outputMimeType('png')).toBe('image/png');
+    expect(outputMimeType('docx')).toBe(
+      'application/vnd.openxmlformats-officedocument.wordprocessingml.document'
+    );
+  });
+
+  it('throws for an unknown target format', () => {
+    expect(() => outputMimeType('made-up-format')).toThrowError(/made-up-format/);
+  });
+});