api rest upload and download files process

How to access file via selected table object.

Hey guys,
I'm new to bonita and today I tried the upload widget by using this example:

So I can upload in one task a pdf-file and download it again in the next task. That works.
But now I want to download the file in another way. My project looks pretty simple, I have 2 forms and one Page.

upload and download files REST API

I´ve intent to create a instance of process (upload and download) using rest API.

params: (dataType:json, contenType:application/json)

{ "processDefinitionId": 7641499513921091240, "variables": ["myDocumentContract": { "filename": "07749533.pdf", "tempPath": "tmp_8964658850826963659.pdf", "contentType": "application/pdf" }]}

But it doesn´t work