Fix empty mime type for ActiveStorage local file service
user
```ruby
# frozen_string_literal: true

module ActiveStorage
  class DirectUploadsController < CustomBaseController
    def create
      blob = ActiveStorage::Blob.create_before_direct_upload!(blob_args)
      render json: direct_upload_json(blob)
    end

    private

    def blob_args
      args = params.require(:blob)
                   .permit(:filename, :byte_size, :checksum, :content_type, :metadata)
                   .to_h
                   .symbolize_keys
      args[:content_type] = 'application/octet-stream' if args[:content_type].blank?
      args
    end

    def direct_upload_json(blob)
      blob.as_json(root: false, methods: :signed_id)
          .merge(direct_upload: { url: blob.service_url_for_direct_upload,
                                  headers: blob.service_headers_for_direct_upload })
    end
  end
end
```
assistant