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>

html/cgi-bin/mail.cgi

@@ -297,7 +297,7 @@ sub testmail {
 		To		=> $mail{'RECIPIENT'},
 		#Cc		=> 'some@other.com, some@more.com',
 		Subject	=> 'IPFire Testmail',
-		Type	=> 'multipart/mixed'
+		Type	=> 'multipart/alternative'
 	);
 
 	### Add parts (each "attach" has same arguments as "new"):
@@ -306,15 +306,6 @@ sub testmail {
 		Data	=> "This is the IPFire test mail."
 	);
 
-	### Add attachment for testing
-	#$msg->attach(
-	#	Type     => 'application/txt',
-	#	Encoding => 'base64',
-	#	Path     => '/var/ipfire/dma/dma.conf',
-	#	Filename => 'dma.conf',
-	#	Disposition => 'attachment'
-	#);
-
 	$msg->send_by_sendmail;
 }