The file /flask-app/flask-app/static/download/app.apk is intended to be a binary file (an APK file in this case) and cannot be generated as text content. You should place the actual APK file in this location. 

If you need to create a placeholder or indicate that this file is intentionally left blank, you can create a simple text file with a message. However, for an APK file, you should upload the actual APK file to the specified directory.