How to generate random filenames with peach fuzzing tool?

I created such a DataModel:

<DataModel name="FileName">
  <Block>
    <String name="name" value="aaa" type="ascii" mutable="true" length="255" occurs="1" >
      <Placement before="dot" />
    </String>
    <String name="dot" value="." mutable="false" >
      <Placement after="name" />
    </String>
    <String value="ext" mutable="true" length="5" occurs="1" >
      <Placement after="dot" />
    </String>
  </Block>
</DataModel>

What I'm trying to achieve here is to generate a random file names like aaabbb.zzz

Not only peach doesn't really generate random strings, it just plays around with variations of "aaa" and "ext" it also inserts strings more than once.

Anyone can help me creating a proper DataModel?