feat: better handling of multipart messages

pkg/message/parser/walker.go

@@ -23,7 +23,23 @@ func newWalker(root *Part) *Walker {
 }
 
 func (w *Walker) Walk() (err error) {
-	return w.root.visit(w)
+	return w.visitPart(w.root)
+}
+
+func (w *Walker) visitPart(p *Part) (err error) {
+	hdl := w.getHandler(p)
+
+	if err = hdl.handleEnter(w, p); err != nil {
+		return
+	}
+
+	for _, child := range p.children {
+		if err = w.visitPart(child); err != nil {
+			return
+		}
+	}
+
+	return hdl.handleExit(w, p)
 }
 
 func (w *Walker) WithDefaultHandler(handler handler) *Walker {
@@ -46,6 +62,14 @@ func (w *Walker) RegisterContentDispositionHandler(contDisp string) *DispHandler
 	return hdl
 }
 
+func (w *Walker) getHandler(p *Part) handler {
+	if dispHandler := w.getDispHandler(p); dispHandler != nil {
+		return dispHandler
+	}
+
+	return w.getTypeHandler(p)
+}
+
 // getTypeHandler returns the appropriate PartHandler to handle the given part.
 // If no specialised handler exists, it returns the default handler.
 func (w *Walker) getTypeHandler(p *Part) handler {