M1.X.X extensions are in .tgz file format with package.xml. The .tgz file name needs to be the same as the package.xml name.
Package.xml name: skywalker/The_Force-1.0.0
.tgz file name: The-Force-1.8.0.tgz
Note: replace underscores with dashes in the .tgz file name to avoid the 'naming conventions' error when uploaded the code package.