Thanks for all your idea and inputs on this!
We have got main part of this implemented and released as part of Filr 3.4 (which was released few hours back)
Feel free to put in a new idea for anything that we may have missed from this idea.
We are planning to add the ability to be notified by email when "shared with" user accesses the file for the first time by providing a tick-box "notify me when recipient accessed shared item" in share dialog in Filr 3.4
Additional capabilities around this area will be added in future.
Any update on this? It seems to be quite simple in implementation and is a feature that's quite obvious to me for a file-sharing application. After all, what good does a shared file do when it doesn't get downloaded?