multipart: Handle multipart/signed

Change-Id: I39742dcce79fc41344c64e5266304c8c120aeb7c
Reviewed-on: https://code.wireshark.org/review/5557
Reviewed-by: Anders Broman <a.broman58@gmail.com>

epan/dissectors/packet-multipart.c

@@ -1053,6 +1053,8 @@ proto_reg_handoff_multipart(void)
             "multipart/form-data", multipart_handle);
     dissector_add_string("media_type",
             "multipart/report", multipart_handle);
+    dissector_add_string("media_type",
+            "multipart/signed", multipart_handle);
 
     /*
      * Supply an entry to use for unknown multipart subtype.