Bug 29330: (QA follow-up) Use passed MIME type

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Amended: tidied.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

C4/Letters.pm

@@ -1253,7 +1253,7 @@ sub _add_attachments {
     foreach my $attachment ( @$attachments ) {
         $message->attach(
             Encode::encode( "UTF-8", $attachment->{content} ),
-            content_type => 'application/octet-stream',
+            content_type => $attachment->{type} || 'application/octet-stream',
             name         => $attachment->{filename},
             disposition  => 'attachment',
         );