message: improve attachment heuristics

Do not consider calender-invitations "attachments"; do mark as
"calendar". Do recognize application/ics as calendar messages.

Update mime-object to expose a message part's disposition.

Change the "is-attachment" heuristic to include inline parts if they
have a filename parameter in their content-disposition.

Note that this doesn't change things radically; the delta is +69 and
-202 for ~6500 attachments.

lib/message/mu-message-part.cc

@@ -195,17 +195,22 @@ MessagePart::looks_like_attachment() const noexcept
 	if (!ctype)
 		return false; // no content-type: not an attachment.
 
-	// we consider some parts _not_ to be attachments regardless of disposition
-	if  (matches(*ctype,{{"application", "pgp-keys"}}))
+	// we consider some parts _not_ to be attachments regardless of
+	// disposition
+	if  (matches(*ctype,{{"application", "pgp-keys"},
+			     {"application", "ics"}}))
 		return false;
 
-	// we consider some parts to be attachments regardless of disposition
-	if  (matches(*ctype,{{"image", "*"},
-			     {"audio", "*"},
-			     {"application", "*"},
-			     {"application", "x-patch"}}))
+	if (is_attachment()) // i.e., as per content-disposition
 		return true;
 
+	// we also consider "inline" parts as attachment, if the
+	// content-disposition has a filename property.
+	if (const auto cdisp{mime_object().content_disposition()}; !!cdisp) {
+		if (const auto& fname{cdisp->parameter("filename")}; !!fname)
+			return true;
+	}
+
 	// otherwise, rely on the disposition
 	return is_attachment();
 }