Because PowerPoint files are large and binary, they don't embed gracefully in JSON. Tailoring instructions expect a templateUrl that points to your PowerPoint template. You may host the file in any cloud storage (eg OneDrive, Google Drive, etc) that can give you a shareable link our servers can access.
Or you may upload PowerPoint and image assets into your Slide Tailor private account storage through the API. When you POST or PUT the binary file to your account endpoint, you'll receive a URL you can then use in tailoring operations. These bash/PowerShell commands upload the hello-me.pptx file created by the hello world example.
You can manage assets you've upload through the API as well. You can list files stored under your account, download or delete files at any time. Your account storage is private: you must include your X-Api-Key in the HTTP headers for all these operations, and no one without that key can access them.
If you're using a free tier, files you've uploaded will be saved for 24 hours. Otherwise they will stay for the lifetime of your subscription, or until you delete them yourself.