feat: add MIME sniffing and output MIME lookup

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

src/mime.js

@@ -0,0 +1,29 @@
+import { fileTypeFromFile } from 'file-type';
+
+const OUTPUT_MIME_TYPES = {
+  jpg: 'image/jpeg',
+  jpeg: 'image/jpeg',
+  png: 'image/png',
+  webp: 'image/webp',
+  gif: 'image/gif',
+  tiff: 'image/tiff',
+  avif: 'image/avif',
+  bmp: 'image/bmp',
+  pdf: 'application/pdf',
+  html: 'text/html',
+  txt: 'text/plain',
+  docx: 'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
+};
+
+export async function detectInputMime(filePath) {
+  const result = await fileTypeFromFile(filePath);
+  return result ?? null;
+}
+
+export function outputMimeType(targetFormat) {
+  const mime = OUTPUT_MIME_TYPES[targetFormat];
+  if (!mime) {
+    throw new Error(`No known MIME type for target format "${targetFormat}"`);
+  }
+  return mime;
+}

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/);
+  });
+});