UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3325.146 Safari/537.36

Steps to reproduce the problem:
1. Open settings and ensure "Download PDF files instead of automatically opening them in Chrome" is DISABLED. This is the default for Chrome. By default, PDFs should be shown in Chrome.

2. Visit a page with links to PDF files. eg.,

<a class="a" download="" href="https://dl.humble.com/exploringarduino.pdf?gamekey=[redacted]&amp;ttl=[redacted]&amp;t=[redacted]">PDF</a>

3. Hold the Alt key and click the link

What is the expected behavior?
When the Alt key is held, the file will be downloaded rather than opened in the browser

What went wrong?
The PDF is opened in the browser whether or not the Alt key is held

Did this work before? Yes Chrome 64

Chrome version: 65.0.3325.146  Channel: n/a
OS Version: OS X 10.13.3
Flash Version: 

The empty string on the "download" attribute in the example is a curious choice on Humble Bundle's part, but this did not previously affect the behavior. I'm not sure how much of a red herring that is. My understanding is <a download="" href...> is synonymous with <a download href...>