List of Users Who Have Access to File

I am creating a plugin that I want to allow someone to select from a drop-down to select someone to assign something. In order to make this work, I need a list of people who have access to the file. Is there a way, in a plugin, to get a list of users or people with view/edit access for a particular file? I can get active users, but that would only show folks who are online and actively in the file at the time.

Thanks.