Setting Filename for Chunked Content

I am using FrameMaker 9 to generate XHTML output from XML files using the DITA-FMx plugin. The XML was converted from unstructured files so each XML file contains several related topics. I'm generating from a bookmap that has a chapter element for each of the XML files. I've then set the chunk attribute for each chapter element to "by-topic". This produces the output I'm looking for where each topic in the chapter generates a separate HTML file. However, the names of the HTML files are all based on the id attributes, which were automatically generated and are not very meaningful for humans. For example, the file names are things like i37215.html and i45200,html. I'm wondering if there is a way to specify the filename (or a prefix) for these chunked files.

If I have a chapter called "Preface" that has two topics within it. When generating, I'd like to produce the following files:

preface.html

preface1.html

preface2.html

A second choice would be to use the title of the topic. So for the same Preface chapter it would be something like:

preface.html

whatcanitdo.html

howdoesitwork.html

Is there a way to control the name of the file that is generated by chunking?