mail.cgi: Fixes bug#13040 - Change multipart/mixed to multipart/alternative

mail.cgi: Fixes bug#13040 - Change multipart/mixed to multipart/alternative

- Thunderbird and Roundcube mail clients presume that any mail with Content Type of
   multipart/mixed has an attachment included rather than actually checking for
   disposition attachment. This means that any mail with multipart/mixed gets the
   attachment icon marked up even though there is no attachment.
- Although this is a problem of the clients involved, in this case the simplest solution
   is to change multipart/mixed to multipart/alternative as the Mail Service test mail only
   sends text without any attachment or other part.
- Confirmed on my vm testbed

Fixes: Bug#13040
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Bernhard Bitsch <bbitsch@ipfire.org>