Write this message body to the file FILE-NAME.
Interactively, the default file name comes from either the message
"Subject" header, or from `rmail-default-body-file'. Updates the value
of `rmail-default-body-file' accordingly. In all uses, if FILE-NAME
is not absolute, it is expanded with the directory part of
`rmail-default-body-file'.
Note that this overwrites FILE-NAME (after confirmation), rather
than appending to it. Deletes the message after writing if
`rmail-delete-after-output' is non-nil.
(fn FILE-NAME)