Maybe a idea for a future release to add a kind of double click action to edit these fields in this dialog.
Currently the double-click opens the clicked attachment in the external assigned application.
The F2 key is Windows standard to enter field edit mode, but you are right that it should be mentioned in the user's guide, especially the need to click previously the column to edit, in order to select the filename or description fields.
Maybe a "edit" item, in the field right click menu, is also a good idea.
Last question, is it also possible to add this "description" using the commandline interface ?
No, but shouldn't be too difficult to add to the Attach command. Now in the todo list.