Ok, I can add more doc for mtime and file_size for their unit and limitation.

I've re-read the gzip spec, the file_name is in ISO 8859-1, the 8-bit Unicode encoding.  For utf-8 string, only the 7-bit ASCII chars are compatible.  I hesitate to use Path since that would give false expectation of storing the whole path.  It really only stores the Path::filename() portion of it.  May be I'll switch to passing in a ~[u8] bytes for the file_name.