presigned post with policy document

Hey,

It's recommended to use presigned uploads with policy to enforce specific limits for stricter control over the uploaded files.

https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-HTTPPOSTConstructPolicy.html

It seems like this is not possible right now with the current provided API: https://async-aws.com/features/presign.html

Did I missed something? 🤔 

