pdf , tiff media type not supported??

Hi,
I'm trying to send fax via rest api using httpclient, but i'm getting this response back

When i try to send a pdf file I get this error:
The request entity's media type 'application/pdf' is not supported for this resource.
When I try to send a tiff file I get this error:
The request entity's media type 'image/tiff' is not supported for this resource.

here's what our account shows:
"AcceptedFormats": [
"image/tiff"
],

Would appreciate any help.
Thanks