CiviCRM APIv4 file custom fields return a bare file id by default; an
extra '.file_name' join is required to get the human-readable filename.
Both the form prefill walk (lib/prefill.ts) and the report walk
(app/api/report/route.ts) now request '<civiField>.file_name' for every
file-type field alongside the primary value, and wrap the prefill into
a { id, file_name } object so downstream UI has both. Falls back to
file_name undefined when the join returns null (eg orphaned id).
The form's FilePriorIndicator already accepts the object shape, so it
now shows the filename inline. The report's FormattedValue gets a
matching case: renders the file_name string if present, falls back to
'Attachment #<id>' when only the id came through.
Joel Brock