How can I use a file as a reference attached to one task? I only have found the "upload button" in the widgets, I can't find the "download button"

yo have the context var in your form page, if you put in a text widget you can watch the store data in it.

{{context}}

In this variable you will have an object called _ref, this object has the url to make your download, this can be do it with the link widget