v6.5.2: feat(files): Group fallback reasons and IDs in mime type backfill output

The `files:fill-mime-type` command now aggregates files whose mime type could not be detected into groups based on the reason for fallback. Instead of listing each individual problematic row, the command outputs a summary for each fallback reason, listing all affected row IDs on a single line. This makes the output more concise and actionable for further processing.