Here is a quick (& dirty) implementation :

In schema

  object :governance_mutations do
    @desc "Upload a file"
    field :upload_file, :string do
      arg :file_data, non_null(:upload)
      resolve(&Resolvers.GovernanceResolver.upload_file/2)
    end

In resolver :

  def upload_file(%{file_data: %Plug.Upload{content_type: _content, filename: orig_filename, path: path_to_filename} = _args}, _) do
    Governances.upload_on_request_type(orig_filename, path_to_filename)
    {:ok, "success"}
  end

Then :

  def upload_on_request_type(orig_filename, path_to_filename) do
    case File.read(path_to_filename) do
        {:ok, contents} ->
                Logger.info("Reading file succeeded")
                %RequestType{}
                |> RequestType.changeset(%{name: "Photo", business_line_id: "1", file_data: contents})
                |> Repo.insert()

                {:ok, "Success"}

        {:error, reason} ->
                Logger.info("File reading error : #{inspect(reason)}")
                {:error, reason}
    end
  end

We need to tell Postgres to accept binary stuff before (otherwise simply copy paste the file elsewhere is filesystemstorage is ok, or even sending it to any bucket).

Thank you @benwilson512 by the way for this really really awesome work