You can include a message with your file upload so your client has context behind what you sent them! You can even use pre-made message templates when sending a file with a message.
To send a message to your client when uploading a file:
Click +Add New in the upper-left corner.
Click File.
In the To field, select Recipient(s) (Contacts) and Account(s).
Select a Year and Month if necessary.
Select Tag(s).
Click on the Message box to type a message to your client. (Note: if you don't type a message an automated message will populate.)
Drag and drop, or browse to add file(s).
Click Upload.