Triggers can be activated when based on a database object's status: before insert, after insert, before update, after update, before delete and after delete. Provided your uploaded files are going to be added to your database, they can be used to activate a trigger.
So the question becomes, how do you manage your uploaded files? Do you have workflow rules you use to manage them before they're accepted into your database? If so, then I'd recommend you add a workflow rule to initiate the desired emails rather than coding a trigger. That would likely be the simplest solution for you since the files presumably need to be added to your database via some method.