ignore application/pgp-signature parts for display

k9mail-library/src/main/java/com/fsck/k9/mail/internet/MessageExtractor.java

@@ -166,6 +166,8 @@ public class MessageExtractor {
                 Html html = new Html(part);
                 viewables.add(html);
             }
+        } else if (part.getMimeType().equalsIgnoreCase("application/pgp-signature")) {
+            // ignore this type explicitly
         } else {
             // Everything else is treated as attachment.
             attachments.add(part);