Figure out how to store metadata purely in s3

[TimMikeladze]

When creating a post policy there is a function available to add metadata to the post policy then gets set on the stored object once the file is uploaded.

I have this flow currently working, however I am not able to figure out how to construct the PUT request that is sent to the post policy url.

I've tried sending metadata as headers or in the form request body, prefixing and not prefixing with the x-amz- headers.

All the approaches ended up in a rejected request.

If this is figured out then next-upload will be able to store metadata for uploaded files without requiring an asset store to be configured.