Feature Request: Support for adding metadata to the post

[zeroasterisk]

Would you consider allowing a user to add metadata to the request (client/server/both/either)?

I have some AWS s3 + lambda workflows which rely on metadata and it existing at the time of upload is more convenient/performant than uploading and then doing a copy&update.

It's actually very simple to add metadata:
http:https://docs.aws.amazon.com/AmazonS3/latest/dev/UsingMetadata.html#object-metadata

The only caveat is that every piece of data you add, has to be added to the policy & signature.

Here's my fork and PR, and the author's solution as a (better) alternative:
SwingDev/s3-browser-direct-upload#3