function reductiveDownloadChain(files){
    return files.reduce((chain,currentFile) => {
        return chain.then(_ => download(currentFile));
    },Promise.resolve())
}

reductiveDownloadChain(mockFiles)
.then(_ => console.log('files were downloaded in reductive 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 reductive chain mode ...(after 9s)