For some reason this has come up and I’d been attending to it in XSL but I really needed it strait out of TCC. Here is the projection that will give that to you.

<quer:projection alias="FileExtension" xmlns:quer="http://www.taleo.com/ws/integration/query">
<quer:substring>
<quer:field path="FileName"/>
<quer:customFunction name="instr">
<quer:field path="FileName"/>
<quer:string>.</quer:string>
<quer:integer>-1</quer:integer>
</quer:customFunction>
<quer:integer>99</quer:integer>
</quer:substring>
</quer:projection>