implemente attachemtn download route

app/attachment/[id]/route.tsx

@@ -0,0 +1,27 @@
+import { fetchBillById } from '@/app/lib/fetchBillById';
+import { notFound } from 'next/navigation';
+
+export async function GET(request: Request, { params:{ id } }: { params: { id:string } }) {
+    const [invoiceID, billID] = id.split('-');
+
+    const bill = await fetchBillById(invoiceID, billID);
+
+    if(!bill?.attachment) {
+        notFound();
+    }
+
+    // convert fileContentsBase64 from Base64 string to binary string
+    const fileContentsBuffer = Buffer.from(bill.attachment.fileContentsBase64, 'base64');
+
+    // convert fileContentsBuffer to format that can be sent to the client
+    const fileContents = new Uint8Array(fileContentsBuffer);
+    
+    return new Response(fileContents, {
+        status: 200,
+        headers: {
+            'Content-Type': "application/octet-stream",
+            'Content-Disposition': `attachment; filename="${bill.attachment.fileName}"`,
+            'Last-Modified': `${bill.attachment.fileLastModified}`
+        }
+    });
+}