A user encountered a response that looks like:
URL: some gibberish
Headers:
Content-Type: application/octet-stream
Content-Disposition: attachment; filename="something.pdf"
In the Chrome extension, the "attachment" content disposition is almost
always ignored (i.e. the PDF Viewer will try to view it anyway). So we
need to fall back to the Content-Disposition header if the URL check is
inconclusive.