How to filter executables using QFileDialog? (Cross-platform solution)

Question

The documentation for QFileDialog.getOpenFileName does not provide any clue on how to filter only executables using a const QString &filter = QString(). Here's the code for my action using PyQt5:

from PyQt5.QtWidgets import QAction, QFileDialog
from PyQt5.QtCore import QDir
from os import path


class OpenSourcePortAction(QAction):

    def __init__(self, widget, setSourcePort, config, saveSourcePortPath):
        super().__init__('&Open Source Port', widget)
        self.widget = widget
        self.setShortcut('Ctrl+O')
        self.setStatusTip('Select a source port')
        self.triggered.connect(self._open)
        self.setSourcePort = setSourcePort
        self.config = config
        self.saveSourcePortPath = saveSourcePortPath

    def _open(self):
        options = QFileDialog.Options()
        options |= QFileDialog.DontUseNativeDialog
        fileName, _ = QFileDialog.getOpenFileName(
            self.widget, "Select a source port", self.config.get("sourcePortDir"), "Source Ports (gzdoom zandronum)", options=options)
        if fileName:
            self.saveSourcePortPath(fileName)
            self.setSourcePort(fileName)

On linux, naturally, I have no file extensions for executables, but I need to filter .exe extensions on windows (Which I intend to provide a version for). Also, there's no overloaded method that allows QDir::Executable. How can I use QFileDialog.getOpenFileName while filtering only executables, no matter which platform it's running on?