"can we extract the attachments from the submitted form?"
Yes. Look at the Attachments property of the processFormSubmission operation of the Forms service.
"If yes, in which format the attachments will be extracted?"
They will be extracted in a list object of subtype "document". The document object is an Adobe proprietary data type that can contain pretty much anything. In your case it'll most likely contain a byte representation of your documents.
Within your Assign Task step in Workbench, go into the "Attachments and Notes" section and ensure that "SHow attachment window for this task" is checked should you wish to allow attachment adding. If the user step you're attempting to enable attachments for is actually your TaskManager endpoint, then you need to go into your service within AdminUI where you'll find a checkbox reading something along the lines of "Allow attachment adding".
I'm a bit confused as to what you are trying to accomplish. Could you clarify what you would like to do for me?
If either of you have questions, please feel free to email me directly!