Topic: Multiple-upload files with ActiveAdmin

I'm trying to create a form that has 3 models (1 for the actual form, 1 for other entries and another one for the uploaded archives). I'm using ActiveAdmin to accomplish that, but ActiveAdmin don't have a documentation for that and I don't have a clue of how to do it.

How can I create a multi-upload file input and add that through my Files model inside ActiveAdmin?

I should have:

Validates if the file is a PDF
Validates his size
Submit everything together with the form ( if the form has an error, don't upload but keep the files inside the input )

Re: Multiple-upload files with ActiveAdmin

Anyone?