Slingshot user metadata

I use Slingshot to upload files to AWS S3. Along with the uploaded file I need to store metadata about the file e.g. the name of the user who uploaded it. Looking on the AWS S3 site it seems you do this by setting some http headers when you upload the file - however I can’t work out how I would specify these for slingshot.

Does anyone one used Slingshot and set user metadata when uploading files? If so, can you please let me know what I need to set.