Set filename of rfc822 attachments when it's absent for Outlook

Outlook on Windows doesn't include filename when attaching .msg files in
generated emails, which broke RT's logic to render them as downloadable
attachments.

This commit fixes it by setting the filename to the subject header of
the .msg file plus .eml suffix, which is also consistent with gmail's
behavior.

Author: sunnavy

lib/RT/EmailParser.pm

@@ -675,7 +675,12 @@ in it.  it's cool to have a 'text/plain' part, but the problem is the part is
 not so right: all the "\n" in your main message will become "\n\n" :/
 
 this method will fix this bug, i.e. replaces "\n\n" to "\n".
-return 1 if it does find the problem in the entity and get it fixed.
+
+Outlook on Windows has another weird behavior that it drops the filename
+when attaching .msg files. This method sets the absent filename to the
+Subject header of attached .msg files with suffix ".eml".
+
+return 1 if it finds some problems above in the entity and get it fixed.
 
 =cut
 
@@ -686,6 +691,7 @@ sub RescueOutlook {
 
     return unless $mime && $self->LooksLikeMSEmail($mime);
 
+    my $changed;
     my $text_part;
     if ( $mime->head->get('Content-Type') =~ m{multipart/mixed} ) {
         my $first = $mime->parts(0);
@@ -727,15 +733,46 @@ sub RescueOutlook {
                 $io->close;
                 $RT::Logger->debug(
                     "Removed extra newlines from MS Outlook message.");
-                return 1;
+                $changed = 1;
             }
             else {
                 $RT::Logger->error("Can't write to body to fix newlines");
             }
         }
     }
 
-    return;
+    # Fix the absent filename of rfc822 attachments
+    for my $part ( $mime->parts ) {
+        my $head = $part->head;
+        my $type = $head->get('Content-Type') // '';
+
+        my $disposition = $head->get('Content-Disposition') // '';
+        if (   $type =~ m{message/rfc822}
+            && $type        !~ /name=\S/
+            && $disposition =~ /attachment/
+            && $disposition !~ /filename=\S/ )
+        {
+            my $subject;
+            for my $line ( @{ $part->body } ) {
+                if ( $line =~ /^Subject:\s*(.*\S)/ ) {
+                    $subject = $1;
+                    last;
+                }
+            }
+
+            if ($subject) {
+                $disposition =~ s!\s+$!!;
+                $head->replace( 'Content-Disposition', qq{$disposition; filename="$subject.eml"} );
+                $changed = 1;
+            }
+            else {
+                $RT::Logger->warning("Can't find email subject when replacing missing filename in Outlook attachment");
+            }
+        }
+
+    }
+
+    return $changed;
 }
 
 =head1 LooksLikeMSEmail