A presigned S3 form for uploading a file. POST fields and the file (as file, last) to url as multipart/form-data. Then use fields.key as the cache_id when you create a custom file.
fields
file
url
fields.key
cache_id
@type t() :: %Docspring.Model.UploadPresign{ fields: %{optional(String.t()) => String.t()}, headers: %{optional(String.t()) => String.t()}, method: Docspring.Model.UploadMethod.t(), url: String.t() }