very often the following happend and i think it could be usefull to have somewhere a menuentry or a button "Mark this file as downloaded".
1.I search for a file
2.I start the download
3.After finished download some parts i press "preview" and see the movie is wrong or after some time i notice the file is not complete somewhere.
4.I press "Cancel" in the context menu.
5.On day later i search for the same file again
6.Back to step 2 again, because i cant memorize the filename and the color in the search pane is "Not downloaded".