Hi,

I found that certain kinds[1] of Japanese mails (like the
attached text part) saved in a nnml group get corrupted when
viewing.  The root cause is this:

(defvar nnmail-file-coding-system 'undecided
  "Coding system used in nnmail.")

The default value used to be `raw-text'.
Because `nnml-file-coding-system' inherits it, when viewing
an article from a file `nnmail-find-file' reads and decodes
the contents, and then `gnus-display-mime'[1] decodes it again.
IIUC `gnus-display-mime' expects a raw article, so any back end
should not decode the first fetched data, I think.  This helps:

(setq nnml-file-coding-system 'raw-text)

Note that the backlog or the `gnus-original-article-buffer'
keeps an corrupted article, so (setq gnus-keep-backlog nil) and
each time killing the original buffer will make it easy to debug.

[1] a single text/plain part with format=flowed spec, or
    a multipart message containing a text/plain part, etc.
[2] The default value of `gnus-display-mime-function'.

Regards,