I’m sure this is too little too late, but in case anyone else has the same question, I think the best thing to do, if you need to save that file to S3 or a database, is to turn it into an upload plug. (Stole this from somewhere I can’t remember)

  def get_plug(data) do
    {:ok, binary} = Base.decode64(data)

    with {:ok, path} <- Plug.Upload.random_file("upload"),
         {:ok, file} <- File.open(path, [:write, :binary]),
         :ok <- IO.binwrite(file, binary),
         :ok <- File.close(file) do
      {:ok, %Plug.Upload{path: path}}
    end
  end

That returns the struct you would have gotten had you uploaded a file directly via form-data (though it will be missing mimetype and filename).