Bad Request While Trying to create transient document in adobe using REST AP

I am trying to create a transient document in adobe sign using rest api.Following is my code snippet:

Http h = new Http(); 
HttpRequest req = new HttpRequest();req.setEndpoint('https://api.in1.echosign.com/api/rest/v6/transientDocuments');req.setMethod(postMethod); 
req.setHeader('Content-Type', 'application/json');blob testFileContent = blob.toPDF('test string it is'); 
req.setBody('{"fileName":"testsign","file":"'+testFileContent+'"}');
String authorizationHeader = 'Bearer ' +acceessToken;req.setHeader('Authorization', authorizationHeader); 
try{
    HttpResponse res = h.send(req);

However I am getting the bad request error in response.Issue maybe due to the tag mismatch with adobe sign requirement, however after a lot of research and trial and error I am not able to find the accurate tags to put in JSON for this API request. Any suggestion and help ?