diff --git a/app/__tests__/ipuzApi.test.ts b/app/__tests__/ipuzApi.test.ts
new file mode 100644
index 00000000..6d33be40
--- /dev/null
+++ b/app/__tests__/ipuzApi.test.ts
@@ -0,0 +1,58 @@
+/**
+ * @jest-environment node
+ */
+
+import { jest } from '@jest/globals';
+import type { NextApiRequest, NextApiResponse } from 'next';
+
+const getPuzzle = jest.fn<(id: string) => Promise<unknown>>();
+
+jest.unstable_mockModule('../lib/serverOnly.js', () => ({ getPuzzle }));
+jest.unstable_mockModule('../lib/converter.js', () => ({
+  exportIpuz: () => new TextEncoder().encode('{}'),
+}));
+
+const { default: handler } = await import('../pages/api/ipuz/[puzzleId].js');
+
+function mockResponse() {
+  const headers: Record<string, string> = {};
+  const res = {
+    headers,
+    statusCode: 0,
+    setHeader: jest.fn((k: string, v: string) => {
+      headers[k.toLowerCase()] = v;
+    }),
+    writeHead: jest.fn((status: number, h: Record<string, string>) => {
+      res.statusCode = status;
+      for (const [k, v] of Object.entries(h)) {
+        headers[k.toLowerCase()] = v;
+      }
+    }),
+    end: jest.fn(),
+    status: jest.fn((s: number) => {
+      res.statusCode = s;
+      return res;
+    }),
+    json: jest.fn(),
+  };
+  return res;
+}
+
+test('ipuz download is not served as application/json', async () => {
+  // Android appends .json to downloads served as application/json, which
+  // turns "puzzle.ipuz" into "puzzle.ipuz.json".
+  getPuzzle.mockResolvedValue({ id: 'abc', t: 'My Puzzle!' });
+  const res = mockResponse();
+
+  await handler(
+    { query: { puzzleId: 'abc' } } as unknown as NextApiRequest,
+    res as unknown as NextApiResponse
+  );
+
+  expect(res.statusCode).toBe(200);
+  expect(res.headers['content-type']).toBe('application/octet-stream');
+  expect(res.headers['content-disposition']).toBe(
+    'inline; filename="My Puzzle.ipuz"'
+  );
+  expect(res.end).toHaveBeenCalled();
+});
diff --git a/app/pages/api/ipuz/[puzzleId].ts b/app/pages/api/ipuz/[puzzleId].ts
index d4c14c3b..83ca22eb 100644
--- a/app/pages/api/ipuz/[puzzleId].ts
+++ b/app/pages/api/ipuz/[puzzleId].ts
@@ -25,6 +25,6 @@ export default async function ipuz(req: NextApiRequest, res: NextApiResponse) {
     'Content-Disposition',
     'inline; filename="' + puzzle.t.replace(/[^\w ]/g, '') + '.ipuz"'
   );
-  res.writeHead(200, { 'Content-Type': 'application/json; charset=utf-8' });
+  res.writeHead(200, { 'Content-Type': 'application/octet-stream' });
   res.end(Buffer.from(exportIpuz(puzzle)));
 }