Make it possible to mop up file imports, by adding some type of marker to imported files. This should be joinable with other maintenance criteria (e.g. copyright templates), and specific to each source wiki.
* Note we already add an HTML comment that says "This file was moved here using FileImporter from …". Example search: https://commons.wikimedia.org/wiki/Special:Search?search=insource:"This+file+was+moved+here+using+FileImporter+from+//en.wikipedia"&ns6=1
* ~~Tags are out because we would need to dynamically create them, and that's – even if it might be possible – not how tags are meant to be used.~~
* A template already exists: https://commons.wikimedia.org/wiki/Template:Transferred_from
* Categories also exist: https://commons.wikimedia.org/wiki/Category:Files_moved_to_Commons_from_other_Wikimedia_projects. But it looks like they are very inconsistently named, and not complete.