In email display, don't suppress text in standalone multipart/related

With PreferRichText enabled, RT's transaction display suppressed a
text/plain email part whose parent was multipart/related, on the
assumption that the part was an alternative representation that would be
superseded by a text/html sibling. When that root is text/plain, and no
text/html part is present, RT rendered an empty transaction even though
the content was available and visible via "View source".

Unlike multipart/alternative, a standalone multipart/related is not an
alternative container (RFC 2387). It has a single root part plus auxiliary
parts the root references, so a text/plain part should not always be
skipped.

multipart/related was originally included in the alternative-handling in
4a38585 to fix "duplicate message" display for the common MUA
structure:

      multipart/alternative
          text/plain          # plain downgrade
          multipart/related
              text/html       # composed rich body
              image/png       # inline embedded image

This commit retains that correct behavior, and now correctly displays
the contents of other multipart/related email that does not follow the
form above.

Fixes: I#38197

Author: cbrandtbuffalo

share/html/Elements/ShowTransactionAttachments

@@ -220,8 +220,20 @@ my $render_attachment = sub {
             # it's a toplevel object
             !$ParentObj
 
-            # or its parent isn't a multipart alternative
-            || ( $ParentObj->ContentType !~ m{^multipart/(?:alternative|related)$}i )
+            # or it isn't part of a multipart/alternative grouping.  A
+            # multipart/related is only an alternative representation when it
+            # is itself nested inside a multipart/alternative (e.g. the rich
+            # side of alternative -> [text/plain, related -> text/html]); a
+            # standalone multipart/related is not an alternative container per
+            # RFC 2387, so its root textual part must always be displayed.
+            || (
+                $ParentObj->ContentType !~ m{^multipart/alternative$}i
+                && !(
+                    $ParentObj->ContentType =~ m{^multipart/related$}i
+                    && $ParentObj->ParentObj
+                    && $ParentObj->ParentObj->ContentType =~ m{^multipart/alternative$}i
+                )
+            )
 
             # or it's of our prefered alterative type
             || (