rack_upload.RdStores the file at path under the given backend, creating a new versioned
entry. This is a low-level generic; most callers should use rack_save()
instead.
rack_upload(backend, path, ...)A rack_id object identifying the newly created version.
rack_save() for the high-level wrapper that serialises R data
before uploading, rack_download() for the complementary download generic.