Phoenix plug change filename in body_params

I am implementing file uploads and use the arc + arc_ecto packages.. after writing my proof of concept I am now trying to handle the file naming in a better way…The user uploads pictures and there is no need to keep the user given file name at all.. therefore my idea is to write a simple plug that overwrites the user uploaded name with an uuid as a file name.. problem is i can’t change the body params like this:

defp rename_file_uploads(conn = %Plug.Conn{:body_params => %{"offer" => %{"pic" => %Plug.Upload{:filename => filename}}}}, _) do
  put_in(conn, [:body_params, "offer", "pic", :filename], gen_unique_filename(filename))
end
defp rename_file_uploads(conn, _) do
  conn
end

what is the right way to do that?