Can I fax a file object via Ruby SDK?

We are integrating RingCentral for faxing and replacing a previous ruby solution. In our solution a PDF (customer invoice) is generated and then faxed, without ever becoming a file on disk.

From what I can glean from the ringcentral-ruby sdk, this isn't possible. Am I missing something? From the examples I'm seeing, its expecting a file on disk, relative to the project. This won't work for us, because its going to be running as a background worker on a server with no local file access.

Is there something I'm missing in the SDK, or some code example of how to get the file data to RingCentral? Or do I need to not use the SDK and code the request by hand?