Getting assemble-middleware-permalinks working

I am trying to get the assemble-middleware-permalinks working on a build, but it doesn't really appear to work the way I want it to.

I have the following in my Gruntfile.coffee:

assemble:
  options:
    layout: 'default.hbs'
    layoutdir: '<%= config.app %>/_layouts'
    partials: '<%= config.app %>/_partials/*.hbs'
    plugins: ['assemble-middleware-permalinks']
    permalinks:
      preset: 'pretty'
  pages:
    files:
      '.tmp/': ['<%= config.app %>/pages/{,*/}*.hbs']

However when running the grunt assemble task, it still shows that it is only generating the following:

Running "assemble:pages" (assemble) task

  writing .tmp/about.html
  writing .tmp/index.html
  assembled 2 files OK

I have the grunt-assemble package installed which uses the 0.5.0 branch of assemble.

I have also tried manually putting in the structure instead of using the preset:

permalinks:
  structure: ':basename/index.html'

Can anyone tell me what I'm doing wrong?