You need to store a few details in your database. To start, you need to create a relationship between users, employees and their managers
With regard to the approval process, you need to store approval information about the uploaded file. This could be represented in a database table field as a bit (null, 0 or 1). Every file that gets uploaded is pending approved by default (null).
When a file gets uploaded, you can send out an email to the user's manager. When the manager logs in, you display the list of that manager's pending file approvals for his/her employees.
The manager can click on the link for the upload and review to disapprove (0) or approve (1). Depending on the selection, you update the field in the DB with the correct value.