The only way to set and get names for snapshots is via the snapshot API. The file names do not correlate at all.
You also can't "overwrite" existing snapshots. You must understand that a snapshot is actually pointer metadata and segment files. When you initiate a snapshot, you're setting up a sequence of events (roughly this):
- Identify all segments used by the indices selected for the snapshot
- Stop background segment merges of identified segments
- Check the remote repository to see if any of those segments have already been copied
- Copy only new segments to the repository
- Update the snapshot metadata so that segments in use have a pointer reference.
- Report snapshot completion (if successful)
- Enable background segment merges of identified segments
This is why a snapshot isn't copying things to a particular file on the remote file system. It's also why you can't "overwrite" a snapshot. The snapshot is technically metadata, and segment files are the actual data.