try passing just '/tmp/file' and you should get
<pre>
file { '/tmp/file':
  ensure => present,
  owner  => 'root',
  group  => 'root',
  mode  => '0644',
}
</pre>

Suggest having the above where we pass the absolute minimum amount of data, as this tests that the defaults get set appropriately, and another one where you specify all of the available options and see that they are all set as well.