File fields: surface a prior-attachment indicator from RHF prefill

The earlier 'Currently on file' indicator hung off readonlyValue, which
is sourced from evalState (only carries current_stage). For file fields
the prefill lives in RHF state, not in evalState, so the indicator
never fired.

Replaced with a FilePriorIndicator subcomponent that subscribes to the
field's RHF value via useWatch and renders a small leaf-50 banner with
a paperclip glyph when a previous attachment is present. Falls silent
the moment the user picks a new file (RHF value becomes a FileList).
Filename is derived from whatever shape Civi returned — bare string,
object with file_name/name/label/filename, or numeric file id (generic
message in that case).

components/fields/FieldRenderer.tsx

@@ -203,16 +203,10 @@ export function FieldRenderer({
 
   // ── File ────────────────────────────────────────────────────────────────
   if (field.type === "file") {
-    const priorName = typeof readonlyValue === "string" ? readonlyValue : "";
-    const hasPrior = priorName.length > 0;
     return (
-      <div className="space-y-1">
+      <div className="space-y-1.5">
         <Label id={id} field={field} />
-        {hasPrior && (
-          <p className="text-xs text-ink-soft" aria-live="polite">
-            Currently on file: <span className="font-medium text-ink">{priorName}</span>
-          </p>
-        )}
+        <FilePriorIndicator control={control} name={field.name} />
         <input
           id={id}
           type="file"
@@ -363,6 +357,68 @@ function CurrencyPreview({
   );
 }
 
+/**
+ * For file fields: detects whether a previously-uploaded attachment is
+ * carried in this field's prefill value (RHF state) and surfaces a small
+ * banner with a paperclip glyph. Falls silent once the user picks a new
+ * file (RHF value becomes a FileList) so it doesn't contradict their
+ * fresh upload. Filename is derived from whatever shape Civi returned —
+ * a bare string filename, an object with `file_name`/`name`/`label`, or
+ * a numeric file id (in which case we render a generic message).
+ */
+function FilePriorIndicator({
+  control,
+  name,
+}: {
+  control: Control<FieldValues>;
+  name: string;
+}) {
+  const value = useWatch({ control, name });
+  if (value === null || value === undefined || value === "") return null;
+  // A FileList means the user has just picked a new file — they don't
+  // need a reminder about what *used* to be on file.
+  if (typeof FileList !== "undefined" && value instanceof FileList) return null;
+
+  let filename: string | null = null;
+  if (typeof value === "string") {
+    filename = value;
+  } else if (typeof value === "object" && value !== null) {
+    const o = value as Record<string, unknown>;
+    const cand = o.file_name ?? o.name ?? o.label ?? o.filename;
+    if (typeof cand === "string") filename = cand;
+  }
+
+  return (
+    <div
+      role="status"
+      aria-live="polite"
+      className="flex items-start gap-2 rounded-md border border-rule-soft bg-leaf-50/60 px-3 py-2 text-xs"
+    >
+      <svg
+        aria-hidden
+        viewBox="0 0 16 16"
+        fill="none"
+        stroke="currentColor"
+        strokeWidth="1.5"
+        strokeLinecap="round"
+        strokeLinejoin="round"
+        className="mt-[1px] h-3.5 w-3.5 flex-shrink-0 text-leaf-700"
+      >
+        <path d="M11.5 4.5 L6 10 a2 2 0 1 0 2.83 2.83 L13.5 7.5 a3.5 3.5 0 0 0 -4.95 -4.95 L3.5 7.5" />
+      </svg>
+      <span className="text-ink-soft leading-snug">
+        Attachment on file
+        {filename ? (
+          <>
+            : <span className="font-medium text-ink break-all">{filename}</span>
+          </>
+        ) : null}
+        . Choose a new file below to replace it, or leave blank to keep it.
+      </span>
+    </div>
+  );
+}
+
 function Label({ id, field }: { id: string; field: FieldConfig }) {
   return (
     <label htmlFor={id} className="block text-sm font-medium text-ink">