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

* fix missing webhook post attachments

* make ProcessSlackAttachments return a new slice instead of modifying it

app/webhook_test.go

@@ -0,0 +1,47 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"testing"
+
+	"github.com/mattermost/platform/model"
+	"github.com/mattermost/platform/utils"
+)
+
+func TestCreateWebhookPost(t *testing.T) {
+	th := Setup().InitBasic()
+	defer TearDown()
+
+	enableIncomingHooks := utils.Cfg.ServiceSettings.EnableIncomingWebhooks
+	defer func() {
+		utils.Cfg.ServiceSettings.EnableIncomingWebhooks = enableIncomingHooks
+		utils.SetDefaultRolesBasedOnConfig()
+	}()
+	utils.Cfg.ServiceSettings.EnableIncomingWebhooks = true
+	utils.SetDefaultRolesBasedOnConfig()
+
+	hook, err := CreateIncomingWebhookForChannel(th.BasicUser.Id, th.BasicChannel, &model.IncomingWebhook{ChannelId: th.BasicChannel.Id})
+	if err != nil {
+		t.Fatal(err.Error())
+	}
+	defer DeleteIncomingWebhook(hook.Id)
+
+	post, err := CreateWebhookPost(hook.UserId, hook.TeamId, th.BasicChannel.Id, "foo", "user", "http://iconurl", model.StringInterface{
+		"attachments": []*model.SlackAttachment{
+			&model.SlackAttachment{
+				Text: "text",
+			},
+		},
+	}, model.POST_SLACK_ATTACHMENT)
+	if err != nil {
+		t.Fatal(err.Error())
+	}
+
+	for _, k := range []string{"from_webhook", "attachments"} {
+		if _, ok := post.Props[k]; !ok {
+			t.Fatal(k)
+		}
+	}
+}