Prefill file fields with their file_name joined from CiviCRM

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.

app/api/report/route.ts

@@ -158,15 +158,26 @@ export async function GET(req: NextRequest) {
   const orgId = orgs[0].contact_id_b;
 
   // Org name + activity walk + option groups, in parallel.
+  // For file-type fields, also request the joined `.file_name` so the
+  // report can surface a human-readable filename rather than the raw
+  // file id that APIv4 returns by default.
   const civiFieldNames = Array.from(
     new Set(allFields.map((f) => f.civiField).filter((f): f is string => !!f)),
   );
+  const fileFieldRefs = Array.from(
+    new Set(
+      allFields
+        .filter((f) => f.type === "file" && f.civiField)
+        .map((f) => `${f.civiField!}.file_name`),
+    ),
+  );
   const select = [
     "id",
     "activity_date_time",
     "subject",
     ACTIVITY_STAGE_FIELD,
     ...civiFieldNames,
+    ...fileFieldRefs,
   ];
 
   const [orgRes, activityRes, options] = await Promise.all([
@@ -227,10 +238,15 @@ export async function GET(req: NextRequest) {
     for (const row of rows) {
       const v = row[f.civiField];
       if (v === null || v === undefined || v === "") continue;
+      let value: unknown = v;
+      if (f.type === "file") {
+        const fname = row[`${f.civiField}.file_name`];
+        value = { id: v, file_name: typeof fname === "string" ? fname : undefined };
+      }
       entries.push({
         activityId: row.id,
         date: row.activity_date_time,
-        value: v,
+        value,
       });
     }
     if (entries.length > 0) fieldHistory[f.name] = entries;