422 Unprocessable Entity error uploading attachment

Using the latest .Net Zendesk API version 3.12.0, I'm getting the

"The remote server returned an error: (422) Unprocessable Entity.Error content: {"error":"AttachmentUnprocessable","description":Attachment file could not be processed."}

I'm passing in a pdf file as "attachment".  It's failing on the UploadAttachment, but I'm passing in the file as a ZenFile.  Here's the code portion that is failing.

ZendeskApi api = new ZendeskApi("https://domain.zendesk.com/api/v2", "user", "password");

                    ZenFile file = new ZenFile
                    {
                        FileName = attachment                        
                    };
                    api.Attachments.UploadAttachment(file);

What am I missing to get this error?