The supported file formats for sending faxes are PDF, TIFF, Postscript and plain text. This applies to all forms of sending faxes (Web API, Windows client software, T.37, and using the web interface on the server itself).
The first example on the API page under “Scheduling a Fax” shows the example XML posted to the web server to send a fax. Look for the <schedule_fax> element. The attachment must be base64-encoded and included in the XML. The example shown has a very small attachment to keep it simple to display.
If you scroll down farther on the page, you will find descriptions of the XML elements using embedded comments. This shows all of the options that are available, including attachment types:
application/pdf
application/postscript
image/tiff
text/plain
Also, I reformatted all of the XML examples on the API page so they are easier to read.
Jesse