Allow null in Options.fileFilter.callback first parameter

types/multer/index.d.ts

@@ -36,7 +36,7 @@ declare namespace multer {
             headerPairs?: number;
         };
         /** A function to control which files to upload and which to skip. */
-        fileFilter?: (req: Express.Request, file: Express.Multer.File, callback: (error: Error, acceptFile: boolean) => void) => void;
+        fileFilter?: (req: Express.Request, file: Express.Multer.File, callback: (error: Error | null, acceptFile: boolean) => void) => void;
     }
 
     interface StorageEngine {