Upload js files to S3 with wrong content-type

When uploading .js files to Amazon S3 storage, the Content-Type is set to binary/octet-stream instead of application/javascript.
When viewing these js files the browser downloads the files instead of viewing the files.

WinSCP uses whatever you have configured in Windows for those file types.
Either fix your Windows settings or configure a type override in WinSCP:
https://winscp.net/eng/docs/rawconfig#mimetypes