Allows the package manager to download gitlab tarballs from urls such as
https://gitlab.com/<namespace>/<project>/-/archive/<sha>/<project>-<sha>.tar.gz

Such http requests have headers Content-Type=application/octet-stream
and Content-Disposition='attachment; filename="<project>-<sha>.tar.gz"'.
The package manager doesn't yet support these headers. This patch
doesn't attempt to properly parse the content-disposition header.
Instead it checks that it starts with 'attachment;' and ends with
'.tar.gz"'.