Make it possible to upload attachments via API

There is the API, but it is not actually uploading to the s3/minio backend. It will just provide a 200 response with the data needed to do the upload myself (if I understood correctly).

It would be much more intuitive, if an API call to that endpoint would upload the file and actually attach it to the document if a document ID is provided. Otherwise it should just upload the file and return the redirect url. The redirect URL is already provided, but the file is not uploaded, which makes it even less expected.

Another good approach would be to return a usable presigned url, so the client can directly upload using that url, instead of creating it on the importer side.