ERR_NGROK_27247: A Files API upload for provider "PROVIDERID" must be a multipart/form-data request - ngrok documentation

What does ERR_NGROK_27247 mean?

ngrok returns ERR_NGROK_27247 with this message:

A Files API upload for provider "<PROVIDERID>" must be a multipart/form-data request.

Where can I get more help with ERR_NGROK_27247?

Email support@ngrok.com and include the error code.