Support for storage system of type `file` broken

[alahiff]

In simvue/factory/proxy/remote.py it tries to upload a file if url is in the JSON response to the POST to /api/artifacts. However, for the case of a storage system of type file, where you just store references to files, the client shouldn't try to upload a file.

If the url is None the upload shouldn't be attempted. Currently the client tries to upload a file to None which fails of course.