static long getId(final Path path) {
        Objects.requireNonNull(path);
        final String fileName = path.getFileName().toString();
        final int start = fileName.indexOf(FILE_SEPARATOR_CHAR);
        if (start != -1) {
            final int end = fileName.indexOf(".", start);
            if (end != -1) {
                final String fullIdString = fileName.substring(start + 1, end);
                if (fullIdString.length() > 0) {
                    return FsPrefixUtil.dePadId(fullIdString);
                }
            }
        }
        return -1;
    }
We use this rather than the File.separator as we need to be standard
across Windows and UNIX.
stroom-data/stroom-data-store-impl-fs/src/main/java/stroom/data/store/impl/fs/FsPathHelper.java