"fileimporter-delete-summary": "This file is now on {{SITENAME}} (moved with FileImporter)."
"fileimporter-delete-summary": "This file is now on {{SITENAME}} as $file (moved with FileImporter)."
where $file would be a link to the imported file, e.g. [[:c:File:Example.png]] https://commons.wikimedia.org/wiki/Example.png for the WMF cluster.
Note:
Apply the above also for the edit summary, so it's easier to find the file when the original will be deleted
The previous discussion started at T226532#5302656.