Bug 1693156 - Fix content-type of MimePart wrapped in a signed message. r=mkmelin

Should not hardcode the type, but inherit from the original MimePart.

mailnews/compose/src/MimeMessage.jsm

@@ -464,7 +464,7 @@ class MimeMessage {
       if (curPart.parts.length > 1) {
         // Move child parts one layer deeper so that the message is still well
         // formed after crypto encaspsulation.
-        let newChild = new MimeMultiPart("mixed");
+        let newChild = new MimeMultiPart(curPart.subtype);
         newChild.parts = curPart._parts;
         curPart.parts = [newChild];
       }

mailnews/compose/src/MimePart.jsm

@@ -286,6 +286,7 @@ class MimeMultiPart extends MimePart {
    */
   constructor(subtype) {
     super();
+    this.subtype = subtype;
     this._separator = this._makePartSeparator();
     this.setHeader(
       "content-type",