Dave Abrahams wrote:
> I'm not sure why sending is failing either ... but the big mystery here
> is why Gnus/message would decompress the archive.

> Sending...
> XZ uncompressing /tmp/YAC2011.mov.xz...done
> Sending via mail...
> message-send-mail-with-sendmail: Sending...failed with exit value 75

Hm, I don't know why it failed.  But decompression should not be
to blame.  Decompressed data are never passed to m-s-m-w-sendmail.

Decompression is done for only determining the charset of
a compressed part; the charset is added to the Content-Type header
of a part if successfully detected.  This is mainly for Gnus users'
convenience.  Try viewing the part attached to this message using
the `i' command.
(A Japanese text will be displayed in a human-readable way in spite
 of the part that is charset-encoded and compressed.  This is
 convenient when viewing a compressed patch file that contains
 non-ASCII text and is sent as an application/octet-stream part,
 for example.  Currently only tar archives are excluded from
 detecting charset.)

You may want to try another approach.  Good luck.