I'm contemplating the way gnus-mime-copy-part ("c" on a button) sets
up the buffer it shows.

With a random 8-bit application/octet-stream, like r.gz below, viewing
with "c" and then attempting to save with C-x C-s prompts for a coding
system, and it seems even no-conversion doesn't give the right bytes.

r.gz for instance should be 393 bytes, as revealled by an "o" save.
And this is all of course with auto-compression-mode off so the zany
jka-compr stuff doesn't get involved.

Is there some magic gnus-mime-copy-part could or should do to give a
buffer which is save-able?  I guess making it unibyte (before
inserting) would work, but I expect that's not good for viewing proper
text files or stuff.