I append an attachment with C-c C-m f that had been originally encoded
in UTF-8.  Gnus decides to reencode it in Latin-1 for whatever reason.
Attachments should _always_ be a faithful byte-by-byte rendition of the
original file.  Gnus should not tamper with that.  It is acutely
embarrassing if you send "example files" to people in order to let them
check whether they have encoding problems at their site, and the file
arrives with a different encoding.

I'd go so far to state that inline included files should keep the same
encoding.  But at any rate, reencoding an external attachment is plainly
ridiculous.

Gnus v5.13
GNU Emacs 24.3.50.1 (i686-pc-linux-gnu, X toolkit)
 of 2013-01-30 on lola
200 news.gmane.org InterNetNews NNRP server INN 2.5.1 ready (posting ok)
100 Legal commands
  ARTICLE [message-ID|number]
  AUTHINFO USER name|PASS password|GENERIC program [argument ...]
  BODY [message-ID|number]
  CAPABILITIES [keyword]
  DATE
  GROUP newsgroup
  HDR header [message-ID|range]
  HEAD [message-ID|number]
  HELP
  IHAVE message-ID
  LAST
  LIST [ACTIVE [wildmat]|ACTIVE.TIMES [wildmat]|DISTRIB.PATS|DISTRIBUTIONS|HEADERS [MSGID|RANGE]|MODERATORS|MOTD|NEWSGROUPS [wildmat]|OVERVIEW.FMT|SUBSCRIPTIONS]
  LISTGROUP [newsgroup [range]]
  MODE READER
  NEWGROUPS [yy]yymmdd hhmmss [GMT]
  NEWNEWS wildmat [yy]yymmdd hhmmss [GMT]
  NEXT
  OVER [range]
  POST
  QUIT
  STARTTLS
  STAT [message-ID|number]
  XGTITLE [wildmat]
  XHDR header [message-ID|range]
  XOVER [range]
  XPAT header message-ID|range pattern [pattern ...]
Report problems to <usenet@ger.gmane.org>.
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382 Begin TLS negotiation now
100 Legal commands
  ARTICLE [message-ID|number]
  AUTHINFO USER name|PASS password|GENERIC program [argument ...]
  BODY [message-ID|number]
  CAPABILITIES [keyword]
  DATE
  GROUP newsgroup
  HDR header [message-ID|range]
  HEAD [message-ID|number]
  HELP
  IHAVE message-ID
  LAST
  LIST [ACTIVE [wildmat]|ACTIVE.TIMES [wildmat]|DISTRIB.PATS|DISTRIBUTIONS|HEADERS [MSGID|RANGE]|MODERATORS|MOTD|NEWSGROUPS [wildmat]|OVERVIEW.FMT|SUBSCRIPTIONS]
  LISTGROUP [newsgroup [range]]
  MODE READER
  NEWGROUPS [yy]yymmdd hhmmss [GMT]
  NEWNEWS wildmat [yy]yymmdd hhmmss [GMT]
  NEXT
  OVER [range]
  POST
  QUIT
  STARTTLS
  STAT [message-ID|number]
  XGTITLE [wildmat]
  XHDR header [message-ID|range]
  XOVER [range]
  XPAT header message-ID|range pattern [pattern ...]
Report problems to <usenet@ger.gmane.org>.
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-- 
David Kastrup