Comment 2 for bug 1784464

When we added Ubuntu Core support we allowed MAAS to accept any DD format supported by Curtin. So Ubuntu Core can deploy any using any DD format, custom images must be a tgz, dd-tgz, or dd-xz, and Windows must be a dd-tgz. We don't validate the format type on upload so you can upload a custom dd-gz image but it won't deploy. The fix is pretty simple, just check for the other formats.