How does a Collaborator know when a file has been added?

You will need to message a Collaborator if you want them to know when a file has been added or something else has changed inside the task.

Since a collaborator has no responsibilities inside a Task, they will not receive notifications when a file is uploaded or anything else has changed. A message will need to be sent to let them know.  LifeStarr is designed this way so that collaborators are not being bothered with notification noise.  The access of a collaborator is more for them to be able to look at the task whenever they want to be updated.