def presigned_upload
        # ${filename} is a placeholder that S3 will automatically replace with the
        # original filename.
        storage_path = generate_storage_path('${filename}')

        presigned_post = s3_bucket.
          presigned_post(key: s3_object_key(storage_path))

        {
          storage_path: storage_path,
          url: presigned_post.url,
          fields: presigned_post.fields
        }
      end