Custom Precompiler For Certain Extensions Only

Is there a way to configure the Texstudio build process so that a custom precompile command is run only for files with a certain extension? In other words, if the extension is .gtex I want the build process to be:

foo.gtex –precompile–> foo.tex –xelatex–> foo.pdf

But if it’s just .tex it should be:

foo.tex –xelatex–> foo.pdf

I’ve successfully set up the command run the precompiler in the configuration screen and it works, but it tries to run the precompiler on every build, not just .gtex files.