You are right, it was inside a form.
I minimized the example a bit too much and have added the form to the initial post.
phx-change was missing as you mentioned. Adding it to the form now updates the socket data correctly, so it fixed one of my two issues 
Now that you mention it, I recognized this is actually mentioned in the docs 
Important: You must bind
phx-submitandphx-changeon the form.
However, Chrome still does not work at all, as no file browser is opened on button click.
