Update openapi.multipart.ts

Add global namespace for typing DefinitelyTyped/DefinitelyTyped#18569 (comment)

src/middlewares/openapi.multipart.ts

@@ -74,15 +74,15 @@ function error(req: OpenApiRequest, err: Error): ValidationError {
     // - 413 ( Request Entity Too Large ) : Too many parts / File too large / Too many files
     // - 400 ( Bad Request ) : Field * too long / Too many fields
     // - 500 ( Internal Server Error ) : Unexpected field
-    const multerError: multer.MulterError = err;
+    const multerError: Express.Multer.MulterError = err;
     const payload_too_big = /LIMIT_(FILE|PART)_(SIZE|COUNT)/.test(multerError.code);
     const unexpected = /LIMIT_UNEXPECTED_FILE/.test(multerError.code);
     const status = (payload_too_big) 
           ? 413 
           : (!unexpected)
               ? 400
               : 500;
-    return validationError(500, req.path, err.message);
+    return validationError(status, req.path, err.message);
   } else {
     // HACK
     // TODO improve multer error handling