The possible file extensions for standard writers (image, surface and point-set) are hard-coded (inside functions) in QmitkIOUtil (or former CommonFunctionality).

The default extensions set in mitkIOUtil like this:
const std::string IOUtil::DEFAULTIMAGEEXTENSION = ".nrrd";
const std::string IOUtil::DEFAULTSURFACEEXTENSION = ".stl";
const std::string IOUtil::DEFAULTPOINTSETEXTENSION = ".mps";

We could also do this for the possible extensions, but it is unclear where exactly.