title: Upload big file to S3 without passing through server
body: <p>I'm developing a web app that allows users to upload big files to the server and I'm using S3 to store those files. In order to be able to upload files of any size, I'm using <code>s3manager.Uploader</code> to simplify doing the multipart upload.</p>
  
  <p>Because some of the files are going to be big (~15gb) I wanted to know if it's possible to do something like the presigned URLs, but for multipart uploads? I want to avoid having to upload the data to my server and then upload it to S3. Ideally I would want an URL where I can just direct a <code>POST</code> in a html form and avoid using my web server as a proxy for the files.</p>