[MM-41998] Channel Info RHS: display number of files for a channel (#19822)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/apitestlib.go

@@ -709,6 +709,33 @@ func (th *TestHelper) CreateMessagePost(message string) *model.Post {
 	return th.CreateMessagePostWithClient(th.Client, th.BasicChannel, message)
 }
 
+func (th *TestHelper) CreatePostWithFiles(files ...*model.FileInfo) *model.Post {
+	return th.CreatePostWithFilesWithClient(th.Client, th.BasicChannel, files...)
+}
+
+func (th *TestHelper) CreatePostInChannelWithFiles(channel *model.Channel, files ...*model.FileInfo) *model.Post {
+	return th.CreatePostWithFilesWithClient(th.Client, channel, files...)
+}
+
+func (th *TestHelper) CreatePostWithFilesWithClient(client *model.Client4, channel *model.Channel, files ...*model.FileInfo) *model.Post {
+	var fileIds model.StringArray
+	for i := range files {
+		fileIds = append(fileIds, files[i].Id)
+	}
+
+	post := &model.Post{
+		ChannelId: channel.Id,
+		Message:   "message_" + model.NewId(),
+		FileIds:   fileIds,
+	}
+
+	rpost, _, err := client.CreatePost(post)
+	if err != nil {
+		panic(err)
+	}
+	return rpost
+}
+
 func (th *TestHelper) CreatePostWithClient(client *model.Client4, channel *model.Channel) *model.Post {
 	id := model.NewId()