If you would like the files in the Matrix locker to be viewable to all AEs who are sharing the accounts, please make sure that before you upload the files into Matrix Locker by clicking on Upload a File button, you select the account owner’s name from the Viewing Account for: drop down list.
For below example, "Matrix, Manager" is the owner of the account “AC Prospect”. Whoever uploads the files into Matrix Locker and wants them to be available to all AEs who are sharing the account, they need to make sure that they select the account owner name (Matrix, Manager) from the Viewing Account for: drop down list. So after the file is uploaded, it will automatically be assigned to the account owner's name(Matrix, Manager).