create order -> upload bill

lib/core/constants/api_constants.dart

@@ -227,6 +227,12 @@ class ApiConstants {
   /// Returns: { "message": { "qr_code": "...", "amount": null, "transaction_id": "...", "bank_info": {...} } }
   static const String generateQrCode = '/building_material.building_material.api.v1.qrcode.generate';
 
+  /// Upload file (bill/invoice/attachment) (requires sid and csrf_token)
+  /// POST /api/method/upload_file
+  /// Form-data: { "file": File, "is_private": "1", "folder": "Home/Attachments", "doctype": "Sales Order", "docname": "SAL-ORD-2025-00058-1", "optimize": "true" }
+  /// Returns: { "message": { "file_url": "...", "file_name": "...", ... } }
+  static const String uploadFile = '/upload_file';
+
   /// Get user's orders
   /// GET /orders?status={status}&page={page}&limit={limit}
   static const String getOrders = '/orders';