Description of problem:
URLs without an “.rpm” suffixes won’t be downloaded by DNF even when the URL points to an RPM file available on the web.

Version-Release number of selected component (if applicable):
dnf-2.5.1-1.fc26

How reproducible:
Every time.

Steps to Reproduce:
1. dnf install https://spideroak.com/release/spideroak/rpm_x64

Actual results:
Doesn’t try to download, verify, nor install the RPM package. (Append .rpm to the URL and DNF will try to download it, but the server will then return a 404.)

Expected results:
Should attempt to download the URL to see if its an RPM or not before dismissing it as an installation candidate.

Additional info:
Note that the `Content-Disposition: attachment; filename=SpiderOakONE.6.3.1.1.x86_64.rpm` HTTP response header gives a suggested filename for the download.