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John Wangleft a comment
It's currently limited to just PDF at the moment which may be good for many use cases. For more more file support, check out the RingCentral fax API which supports sending 29 different file types including PDF, TIFF, DOCX, DOC, XLSX, XLS, RTF, HTML, and XML. - https://developers.ringcentral.com/api-docs/latest/index.html#!#RefFaxMessages.html
Twilio Programmable FaxAPI for sending and receiving faxes