Change input method of "Description field" in "File" type
One way to do it without coding might be to create custom entities with Entity Construction Kit and insert them to your content type using Inline Entity Form. Using ECK, you can add the dropdown/autocomplete field and the file field. 

You may then need to use something like Automatic Entity Label to prevent the user from needing to enter description/title themselves (The site won't allow me to insert a third URL here, but an internet search for "automatic entity label drupal module" will bring it up).