How to Send pdf from Salesforce to Echosign through REST API?

[vignex]

I'm trying to send PDF generated from Visualforce page and through Apex REST I'm trying to invoke Echosign REST API. But while invoking "/transientDocument" URL, it says Unsupported content type. Any help on how to construct multipart/form-data in Salesforce?

[SimonATS]

I'm afraid it's a bit beyond me, but why not use the ready-made Echosign for Salesforce application you can download from the app exchange?