>>> "Stefan" == Stefan Monnier <monnier@iro.umontreal.ca> writes:

   >> It seems I misunderstood your original report: you wanted Z on a .tgz
   >> file to only uncompress it?  That indeed has been changed in Emacs 26,
   >> for consistency with a .tar.gz file.  I don't think it's a good idea
   >> to have the previous subtle difference back.

   > Agreed.

   >> I do think that Z on a compressed Tar archive, be it a .tar.gz or .tgz
   >> file, should not by default unpack the archive.  We could have a
   >> special prefix arg to request that, and by default we should just
   >> uncompress the file.  But that's a different issue.

   > FWIW, I consider ".tar.gz" (or "tar.lz", ...) as the archive format
   > (rather than as a combination of tar and compression): since the tar
   > format does not support random access anyway there's very little benefit
   > to having it uncompressed (unless the content can't be compressed
   > e.g. because it's already compressed).

Unfortunately some sites require to upload files in tar format and not in
tar.gz format. 

If I understand the current situation correctly, there is no key
combination or interactive function which would allow to just uncompress
a tgz (tar.gz) file? The only option is to use the
dired-do-shell-command?