Issuing the request from the example returns a assembly result that is currently being executed. Then i have to wait and ask for the result after some time.
Is there any way to make it synchronous?
I don't do anything special with how the assemblies are sent or received. You'd need to refer to Transloadit's documentation to a synchronous upload. BTW, I don't believe they actually support synchronous stuff like this. It appears they either redirect when done or will ping you when it's all done: http://transloadit.com/docs/notifications-vs-redirect-url