I work for Maytech and we have a product - Quatrix Personal: httpswww.maytech.net/products-quatrix-personal.html Opens a new window that might work for you..

- It is free (always a bonus)

- It is web based software perfect for sharing files with customers or suppliers

- It has granular controls in place but you only have one user (we have Quatrix Business which is subscription based for multiple users) 

- We provide LDAP authentication (although this is a chargeable module on Quatrix Business)

- We don't currently have an approval workflow - although good idea. Let me mention it to our tech team though:)