I agree with Pranali. You could also try to use the feature on the CLI and see what happens if "web-download" has not been enabled in glance-api.conf, for instance.
Do you have a link to the related tests in Horizon?
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHEA-2019:0045