Had some thinking...
the only reason I am passing through file-storage to send attachments is that ACS Mail Lite currently needs a file to reside into the file-storage to send it as an attachment (as far as I know).
At the time I just wanted the feature up and running, so I didn't care, but you can tell yourself this approach is far from being optimal: I have to save my attachment into the file-storage, send the email, then delete it... and all of this needs to happen in a transaction in order to be safe.
It won't take forever to extend Mail Lite so it can also send attachments taken from a regular file on the server, and this would be very convenient for me.
I will propose you my change to see what you think