PLT-7212: fix missing webhook post attachments (#7011)

* fix missing webhook post attachments

* make ProcessSlackAttachments return a new slice instead of modifying it

model/slack_attachment_test.go

@@ -10,8 +10,8 @@ func TestExpandAnnouncement(t *testing.T) {
 	}
 }
 
-func TestSlackAnnouncementProcess(t *testing.T) {
-	attachments := SlackAttachments{
+func TestProcessSlackAnnouncement(t *testing.T) {
+	attachments := []*SlackAttachment{
 		{
 			Pretext: "<!channel> pretext",
 			Text:    "<!channel> text",
@@ -25,7 +25,7 @@ func TestSlackAnnouncementProcess(t *testing.T) {
 			},
 		}, nil,
 	}
-	attachments.Process()
+	attachments = ProcessSlackAttachments(attachments)
 	if len(attachments) != 1 || len(attachments[0].Fields) != 1 {
 		t.Fail()
 	}