Getting HTTP Error: 400 Bad Request on GIF or video upload on Twitter

I am using Twitter npm package to make a simple app that can upload gif or video to the platform. Getting 400 Bad Request error on the FINALIZE step. I can't understand why am I getting this error. I have structured my code as the example provided by the package and the Twitter docs. There is not much discussion about this error on stack overflow and in Twitter forums, they are saying that if a media file doesn't pass validation then this error is produced but they didn't specify the validations. If someone can explain why is it happening then that would be a great help.

let fs = require('fs')
let path = require('path')

const keys = require('./config')

const client = new Twitter(keys)

let mediaIdString = ''
let mediaId = 0

//  reading file (GIF)
const gifFile = fs.readFileSync(path.join(__dirname, 'memes', 'gif1.gif'))

//  getting file details
const gifFileStat = fs.statSync(path.join(__dirname, 'memes', 'gif1.gif'))
const gifSize = gifFileStat.size
const contentType = 'image/gif'

console.log(`gif size in bytes ==> ${gifSize}`)

//  calling uploadMedia
uploadMedia().then((result) => {
    console.log('UPLOAD SUCCESSFUL')
}).catch((err) => {
    console.log(err)
})

function uploadMedia () {
    //    STEP 1 (INIT)
    return client.post('media/upload', {
        command: 'INIT',
        total_bytes: gifSize,
        media_type: contentType
    }).then((response) => {
        console.log(`INIT \t response ==> ${JSON.stringify(response)}`)

        mediaIdString = response.media_id_string
        mediaId = response.media_id

        console.log(`mediaId ==> ${mediaId}`)

        console.log(`media id string ==> ${mediaIdString}`)

        //  step 2 (APPEND)
        return client.post('media/upload', {
            command: 'APPEND',
            media_id: mediaIdString,
            media_data: gifFile,
            segment_index: 0
        })
    }).then((response) => {
        console.log(`APPEND \t response ==> ${JSON.stringify(response)}`)

        //  STEP 3 (STATUS)
        return client.get('media/upload', {
            command: 'STATUS',
            media_id: mediaIdString
        })
    }).then((response) => {
        console.log(`STATUS \t response ==> ${response}`)

        //  STEP 4 (FINALIZE)
        return client.post('media/upload', {
            command: 'FINALIZE',
            media_id: mediaIdString
        })
    })
}

from Getting HTTP Error: 400 Bad Request on GIF or video upload on Twitter