The http.FileServer documentation says:

As a special case, the returned file server redirects any request ending in "/index.html" to the same path, without the final "index.html".

Based on this, we expect the request in the question to redirect.

The browser follows the redirect to correct URL and the server returns the resource as expected.

The response recorder returns the redirect as is. In other words, the response recorder does not follow the redirect.

Fix one these ways:

• use the correct URL
• use http.TestServer and the net/http client (which follows redirects).
• Write code in your test to follow the redirects.