Add file_name field to QueryResult.

[Frodo45127]

This adds the "file_name" field to QueryResult.

AFAIK, this field is empty in modern games, but it's needed to manage workshop items in older games.

For example, in Total War: Shogun 2, mods are ".pack" files. When uploaded, they get wrapped into a ".bin" file with a random name. When you subscribe to them, the ".bin" file is downloaded, and you need this field to know what the original name of the file was.