Updated: better handling of file extensions

- now more strict (doesn't remove the extension if it is not an
  extension in __extensions__ of the decompressor
- test made
Fixed: stream through ArchivePack doesn't have the right file name

- If an ArchivePack has only one file, the realname should be taken from
  the filename (or name) attribute of the member
- Test updated to check the realname of ArchivePack having only one file