Cannot send fax attachments via javascript API

I have tried unsuccessfully to send fax attachment via the API:

- Tried two different destination faxes

- Tried two different pdf attachments (including simply resending the fax cover sheet RC generates)

- Tried appending the attachement to formData as a Blob and File

- Checked the Contect-Type is set to application/pdf

In all cases the fax is indicated as Sent on the message store and indicates that there are 2 pages to be sent (cover + 1 one-page attachment) but the receiving fax machine only gets the cover page.

Please help - it takes way to long to develop applications based on RingCentral!