Filing is a nightmare which ever way you look at it so you need to make it as simple as possible.
Generally the highest level of division are Clients, Suppliers, Information and Accounts.
These can then be sub-divided as appropriate for the business.
The big hint is to then use the filing names in your filing cabinet, on your computer document filing and your emails inbox. Using the same structure and naming conventions you will find things much easier.
If you can keep an index either on cards or like a family tree to the system then it always acts as a useful aid memoir and can be given to your assistant and you won’t have to train them in your system.