Custom collection actions / change `new instance` buttons according to class

[joepio]

When opening the Files collection, you see a new file button, which opens the form. This form is pretty useless, as uploading files is a different kind of action, using an Endpoint.

I can do a couple of things here:

• The new instance button checks for classes, changes its behavior accordingly. Makes assumptions on parent for file upload. This can also work for documents.
• The collection page checks for classes.