Assuming you are running this on a UNIX-like OS, if you just move the file to a new filename on the same volume, it won’t copy the data (which is slow) but simply create a new file entry for it (fast), and the Phoenix process will not delete it. I have a function that attempts to move the file, and only if that fails then falls back to copy for this exact purpose:

  @doc "Move a file, copy if we must"
  @spec move_file(src_path :: String.t(), dest_path :: String.t()) :: :ok | :error
  def move_file(src_path, dest_path) do
    # move if we can, copy if we have to
    case File.rename(src_path, dest_path) do
      {:error, :exdev} ->
        case File.copy(src_path, dest_path) do
          {:error, _} = error -> error
          _ -> :ok
        end

      {:error, _} = error -> error
      _ -> :ok
    end
  end

Correct, the uploaded temp file is cleaned up when its upload process exits, but this does not occur until you invoke one of the consume_uploaded_* functions for a given upload entry. So you can start uploading automatically and still wait to consume them in your form’s phx-submit handler.