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.

lib/prefill.ts

@@ -39,7 +39,19 @@ export async function loadPrefill(
   activityTypeName = "Org Engagement Submission",
 ): Promise<PrefillResult> {
   const civiSelected = fields.filter((f) => f.civiField);
-  const select = ["id", "activity_date_time", ...new Set(civiSelected.map((f) => f.civiField!))];
+
+  // CiviCRM File custom fields return a file id by default. To surface a
+  // human-readable filename in the prefill (so the prior-attachment
+  // indicator can show it), also request the joined `.file_name` for any
+  // file-type field.
+  const fileFieldRefs = new Set(
+    civiSelected.filter((f) => f.type === "file").map((f) => f.civiField!),
+  );
+
+  const selectSet = new Set<string>(["id", "activity_date_time"]);
+  for (const f of civiSelected) selectSet.add(f.civiField!);
+  for (const ref of fileFieldRefs) selectSet.add(`${ref}.file_name`);
+  const select = Array.from(selectSet);
 
   const res = await civi<ActivityRow>("Activity", "get", {
     select,
@@ -56,10 +68,17 @@ export async function loadPrefill(
   for (const f of civiSelected) {
     for (const row of rows) {
       const v = row[f.civiField!];
-      if (v !== null && v !== undefined && v !== "") {
+      if (v === null || v === undefined || v === "") continue;
+      if (f.type === "file") {
+        const fname = row[`${f.civiField!}.file_name`];
+        out[f.name] = {
+          id: v,
+          file_name: typeof fname === "string" ? fname : undefined,
+        };
+      } else {
         out[f.name] = v;
-        break;
       }
+      break;
     }
   }