From f7c7e547d2cbf08e6c506a69f7174da78584d75e Mon Sep 17 00:00:00 2001
From: Anthony GAEREMYNCK <1@anthony.sh>
Date: Thu, 30 Jul 2026 09:00:45 +0200
Subject: [PATCH] feat: add MIME sniffing and output MIME lookup

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 src/mime.js              |  29 +++++++++++++++++++++++++++++
 test/fixtures/sample.png | Bin 0 -> 68 bytes
 test/mime.test.js        |  27 +++++++++++++++++++++++++++
 3 files changed, 56 insertions(+)
 create mode 100644 src/mime.js
 create mode 100644 test/fixtures/sample.png
 create mode 100644 test/mime.test.js

diff --git a/src/mime.js b/src/mime.js
new file mode 100644
index 0000000..f0c63e7
--- /dev/null
+++ b/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;
+}
diff --git a/test/fixtures/sample.png b/test/fixtures/sample.png
new file mode 100644
index 0000000000000000000000000000000000000000..520b38b7bfbb918d01382f7f5abbd6714eda7eac
GIT binary patch
literal 68
zcmeAS@N?(olHy`uVBq!ia0vp^j3CUx1|;Q0k8}blZci7-kcv6U2@FgO46F={FMr*+
Q0TgENboFyt=akR{0DRC61ONa4

literal 0
HcmV?d00001

diff --git a/test/mime.test.js b/test/mime.test.js
new file mode 100644
index 0000000..bdc5e47
--- /dev/null
+++ b/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/);
+  });
+});