Update multi_add_file_to_scene.py

We had a lot of problems with the loader in maya from the beginning because it was missing namespace logic in the hook. The problem is, that if namespace is not specified, Maya only uses filename as a renaming prefix and locks the reference. This results in broken rigs if there is another nested reference in the rig file, because only clashing objects get this renaming prefix, instead of everything. I've made a simple fix which doesn't tweak the actual namespace, but only takes the base of the filename to make it as generic as it was. I though this might be beneficial to pull into default hook as it really looks like a bug, rather than a feature ;).

We are referencing Alembics at least as often as maya files, so I've also added Alembic referencing into the list of maya supported extensions.

Commit comments:

Added basic namespace logic to add_file_to_maya to explicitly set namespace to the basename of the file without extension. 

Alembic is added to the default list of supported extension as it can be referenced in the same way as standard maya file.