function recursiveDownloadChain(files){
    const nextFile = files.shift();

    if(nextFile){
        return download(nextFile).then(_ => recursiveDownloadChain(files))
    }else{
        return Promise.resolve();
    }
}

recursiveDownloadChain(mockFiles)
.then(_ => console.log('files were downloaded in recursive chain mode'))

// ----- output in console -----

// started downloading file 1 ...(immediately)
// downloaded file 1 in 2000ms ...(after 2s)

// started downloading file 2 ...(after 2s)
// downloaded file 2 in 4000ms ...(after 6s)

// started downloading file 3 ...(after 6s)
// downloaded file 3 in 3000ms ...(after 9s)

// files were downloaded in recursive chain mode ...(after 9s)