Sharepoint - Selecting only PowerPoint files in a CAML query

Try:

<Where><Eq><FieldRef Name='FileExtension'><Value Type='text'>pptx</Value></FieldRef></Eq></Where>

You may use DocIcon to determine the type of file in your CAML Query

<Eq><FieldRef Name="DocIcon"/><Value Type="Computed">pptx</Value></Eq>

Source: Filter file types from list to NOT show certain file types