Add file fields to comment objects [SOLVED]

I am trying to add a new field to a Plone Comment object, specifically I need to add a file. I tried the way of this tutorial that shows how to add new fields to the comment form:

Nevertheless, I can only add the field. No matter what I upload using the form, it doesn't get saved on the comment. But if I add a simple TextLine (like the one in the example), it gets added normally.

I must be missing something but I don't know what. Any idea?

Sorry, I was just looking on the context object instead of on the comment object. I can save files with no problems.